Who wouldn’t grab an income tax refund if they had one coming?

About 940,000 people, it turns out — because they haven’t filed returns for the 2020 tax year, even though they may be due money back for that year. But there’s still close to a month left to file and collect the refunds. The Internal Revenue Service estimates that the typical refund for the people in this group is more than $900.

“There’s money remaining on the table for hundreds of thousands of people who haven’t filed 2020 tax returns,” the I.R.S. commissioner, Daniel Werfel, said in a statement. The deadline for most people to file a return and collect the refunds, which total about $1 billion, is May 17.

Potential refunds range from a typical amount of $761 in Idaho to more than $1,000 in New York and Pennsylvania, the I.R.S. said. (Actual amounts vary depending on the filer’s tax situation.)

Some people may simply have forgotten to file a 2020 return because of “extremely unusual situations” during the pandemic, Mr. Werfel said. Still, said Eric Smith, an I.R.S. spokesman, the number of taxpayers potentially due a refund this year for 2020 is not at a record level. Last year, which was the three-year deadline to file and collect unclaimed refunds from the 2019 tax year — returns that were initially due in the depths of the pandemic in 2020 — nearly 1.5 million people were potentially due refunds, typically about $800.

The I.R.S., too, was affected by the pandemic. The agency struggled to process paper tax returns and correspondence, though the I.R.S. has “virtually eliminated our backlog of unprocessed paper returns” for individual returns, Mr. Smith said.

If you don’t file a 2020 tax return by the cutoff, you’ll lose the refund — and the Treasury Department keeps it. “Why leave that money on the table?” said Tom O’Saben, director of tax content and government relations with the National Association of Tax Professionals.

In general, taxpayers have three years to file a return and claim refunds. (There’s no penalty for failing to file if you’re getting money back.) Tax returns for 2020 were due in 2021, but the filing deadline that year was postponed until May 17 from the usual mid-April date because of the pandemic. So the I.R.S. has also stretched the three-year window for filing those returns by a month.

Some unclaimed refunds may be owed to part-time workers and others who didn’t earn enough money to meet the requirement for filing a return. (The filing threshold in 2020 was $12,400 for single filers and $24,800 for couples filing jointly; higher thresholds applied for those 65 and older.)

Potential refunds could be higher than the I.R.S. estimates because of the effect of tax credits as well as pandemic stimulus payments made in 2020. “The reality is these are conservative” estimates, Mr. Smith said.

Some people may be eligible for recovery rebate credits if they didn’t receive their pandemic stimulus checks — also known as economic impact payments — in 2020. Most eligible people did get their payments or have already claimed them as a tax credit, the I.R.S. said. But if they qualified and didn’t receive the payments, they can file a tax return to get the money as a credit — even if they had minimal or no income. The credit is “refundable,” meaning you can still get the money as a refund even if you don’t owe any tax.

Also, many low- and moderate-income workers may be eligible for the earned-income tax credit, which is also refundable. The earned-income tax credit is based on family income and size. For 2020, the credit was worth as much as $6,660 (for someone with income up to $50,594 and three or more children).

“That’s a substantial amount of money for working families,” Mr. O’Saben said.

Even filers without children are eligible for the credit if their income qualifies, said Andy Phillips, director of H&R Block’s Tax Institute. In 2020, a childless individual with income of up to $15,820 was potentially eligible.

People who have missed filing a tax return for one year have often missed other years as well, said Cynthia Leachmoore, president of the National Association of Enrolled Agents, whose members are federally authorized tax professionals. People may initially fail to file simply because they procrastinated, or perhaps because they owed money, and then find it difficult to get back on track.

“They think it’s insurmountable,” she said. “It becomes a difficult cycle to break.”

If you haven’t filed a return for 2021 or 2022, the I.R.S. may hold your 2020 refund until you do. Also, your refund may be applied to any tax amounts you still owe or used to offset unpaid child support or other past-due federal debt, like student loans.

Be aware that 2020 returns must be filed on paper, whether you do it yourself or use a paid preparer. The I.R.S. accepts electronically filed returns for the current tax season and two years prior, Mr. Smith said. You can find prior-year tax forms online at IRS.gov, or use do-it-yourself software to prepare your return. But you’ll have to print the 2020 return and mail it to the I.R.S.

Ms. Leachmoore advised mailing the return by certified mail with a return receipt requested, so you will have proof that you met the May 17 deadline and that the I.R.S. received your return.

There are some exceptions to the extended May filing deadline, Mr. Smith said, such as military service in a combat zone or a “financial disability” that prevents you from managing your money because of a physical or mental impairment.

But in general, he said, it’s best to file the return by May 17 so you can claim your refund. “The sooner you file,” he said, “the sooner you’ll get your money.”

Here are some questions and answers about late-filed tax returns:

What if I don’t have the tax documents I need to file a 2020 return?

You can ask your employer for W-2 wage statements, businesses for 1099 forms if you worked for them as a contractor, and your bank for interest statements. If you can’t get the documents that way, you can order a free “wage and income” transcript, which shows information from various documents the I.R.S. has received about you, including W-2 wage statements and 1099 forms, using the I.R.S. “Get Transcript” online tool. Since the deadline is fast approaching, “start to do the legwork now,” Mr. O’Saben said.

When can I expect my refund after filing my 2020 return?

Filers shouldn’t expect speedy refunds because paper returns typically take longer to process, tax experts said. “It’s not going to be 21 days, like it is for electronic filing,” said April Walker, lead manager of tax practice and ethics with the American Institute of Certified Public Accountants.

Mr. Smith at the I.R.S. said that if you filed a “complete and accurate” 2020 paper tax return, the refund should be issued in about six to eight weeks from the date the agency received the return.

You can’t track your 2020 refund using the I.R.S. “Where’s My Refund?” service because it can be used only for the current tax season and two years prior, Mr. Smith said. You can, however, create an I.R.S. account, which can be helpful in tracking older refunds, he said.

What if I missed the April 15 deadline to file my 2023 tax return?

It’s smart to file as soon as you can, Mr. Smith said. If you’re owed a refund, you can’t get it until you file. And if you owe tax, you’ll minimize penalties and interest charged. (You can ask for a waiver of penalties, if you have a history of filing on time.)