Windows Phone was great. You don't have to agree with that, and it certainly wasn't the right choice for a lot of people, but the phones that ran Microsoft's OS offered something to people who wanted simple and easy phones to use. Cheaper than an iPhone and clearer than Android, Windows Phone 8 was actually what the world needed, but now it's dead, dead, dead.
Microsoft has announced that it will no longer offer updates for the older devices that run its mobile OS. Users of some phones will be lucky, and able to upgrade - if they haven't already - to Windows 10. This will be good news for some Windows Phone loyalists or course, but realistically the number of these devices sold was very small. Windows Phone might have been a good platform, but it didn't resonate enough with the audience.
However, for now at least, Microsoft isn't out of the Windows Phone game. Devices running Windows 10 can run the same apps as PCs can, in theory, assuming the developers have moved to "universal" apps. For example, the IoT doorbell Ring has a universal app that works on PCs and phones because it's super-easy to support everything. And frankly, with Xbox One also on this codebase the future for Windows Phone could be bright.
But not Windows Phone 8.1. That's dead man.
If you still have a Lumia phone with Windows 8.1 then you may be able to move it to Windows 10 by installing the "Upgrade Advisor" from the app store. Once you open the app it will allow you to agree that you'd like to move to Windows 10. Once you've done that, the phone will then be updatable via the usual settings menu.
Phones which can take Windows 10 are: Lumia 1520, 930, 830, 730, 735, 640, 640XL, 635, 636, 638, 540, 535, 532, 435 and 430.