It's been nearly a year and a half since support ended for Windows Phone 8.1, not that it had even received an update for a couple of years before that. Of course, when support ends, the device still works, although services begin to get shut down. One of those services is the Windows Store. As Microsoft announced in October, the Store for Windows Phone 8.1 is shutting down today, and you can no longer download apps.
The apps you have will continue to work, but if you do a factory reset on your phone, you won't be able to get them back. The only way to continue getting apps on your phone is to upgrade it to Windows 10 Mobile.
That's another issue though, because you actually need an app to get the update, at least the over-the-air update. When Microsoft launched Windows 10 Mobile, it did it for a small subset of devices, mainly those that launched with Windows Phone 8.1 rather than those that upgraded from Windows Phone 8. And it never notified users that there was an upgrade available. You had to know about it, and seek it out yourself by downloading the Upgrade Advisor app.
Luckily, there's a way to still get Windows 10 Mobile, assuming that your phone is supported for the upgrade. You can use the Over-the-Cable Updater tool that Microsoft provides.
Windows 10 Mobile, of course, isn't supported either, but at least you'll have more working services. Most phones that are upgradeable will be able to get up to the Windows 10 Mobile Anniversary Update (version 1607), with the exception of the Microsoft Lumia 640 and 640 XL, which will land on the Windows 10 Mobile Creators Update (version 1703).
Microsoft was supposed to end support for all versions of the OS this month, but it inexplicably extended support for a month at the last minute. This doesn't apply to devices that shipped with Windows Phone 8.1 though, as none of them were eligible for Windows 10 Mobile version 1709.