Americans who waited until the last day to pay their taxes online got an unwelcome surprise: The IRS website to make payments and access other key services went down earlier today.

Now, taxpayers will get a one-day extension, and the filing system is back online.

Electronic filing is the most popular way to file. Earlier in the day, Treasury Secretary Steven Mnuchin told reporters in New Hampshire the IRS was experiencing “a high volume technical issue that impacted the system.”

Pages on the IRS website used to view account information, make a direct payment or set up a payment plan were all not functioning most of the day.

It’s unclear when and why the failure occurred. But it appears, based on a message on the site, that the online payment system became unavailable at 2:50 a.m. ET that morning.

It’s unclear how many people were affected but, by comparison, about 5 million tax returns were filed on the final day of last year’s tax season.

The IRS snafu also caused problems for popular third-party tax preparers such as Turbo Tax and H&R Block. Both said that they will hold onto customer tax returns and file them as soon as the IRS system reopened.

Tax day falls on April 17 this year because April 15 was a Sunday and April 16 was Emancipation Day, a holiday in Washington, D.C.

IRS Acting Commissioner David Kautter testified during a House Oversight Hearing Tuesday that a number of systems are down at the moment and that the agency is working to resolve the issue.

Trump’s top economic adviser Larry Kudlow offered a deadpan reaction when asked about the failure.

“The IRS is crashing? Sounds horrible. Really bad,” he said during a briefing with reporters in West Palm Beach, Florida. “I hope it gets fixed.”

The IRS typically recommends that taxpayers use electronic filing to avoid common mistakes. Online filing is quicker than dropping something in the mail — when the site works, of course.