It's not certain what happened to Direct Pay or when it will be back. IRS Acting Commissioner told those at a hearing that it was "probably" an internal technical glitch rather than a cyberattack. Don't panic too much at this point, then. The agency is trying a "hard reboot" to solve it. And in case you're wondering: Kautter is promising that the IRS won't punish people if the technical failures prevent them from filing on time. If you're truly without options at this point, you'll likely get a reprieve.
Whatever the cause, the failure underscores the challenges of offering such a vital government service online. The IRS' Direct Pay system not only has to cope with tremendous demand from taxpayers (5 million people filed returns at the last moment in 2017), but protect against hacks and brute force denial-of-service attacks. That's a daunting challenge for any organization -- and when the IRS has a less than stellar track record on this front, it's no exception to the rule.
Update: Direct Pay is back up and running, and Treasury Secretary Steve Mnuchin has promised extensions to those affected by the outage. He described it as a "high volume technical issue," although that doesn't shed much more light on the situation.
Update 2: The IRS has granted taxpayers an extra day to file -- you now have until midnight on April 18th. The one-day extension won't require any effort on your part. If you need more time than that, though, you'll still need to ask for it.