Although it's hard to be sure just having your question to go on, the first thing to do is make sure it's not a software problem. The best way to do that would be to restore the phone to factory settings (note that you will lose all of your data and pictures), or you can try starting the phone in safe mode. Here's Verizon's instructions for putting your phone in safe mode.
[quote|format=featured]***Samsung Galaxy A12 - Restart in Safe Mode***
# Press and hold the. Power / Side button. until the 'Power off' prompt appears on the screen then release.
# Touch and hold. Power off. ...
# To confirm, tap. Safe mode. ...
# With Safe Mode enabled, ensure Airplane Mode is turned off then test device and app functionality.[/quote]
If the power button is still misbehaving, then you most likely will have to replace it. I didn't find any step by step iFixit-style guides for your exact phone, but I did locate on for the A12 Nacho, which is a slightly newer version of yours, but as far as I can tell the replacement procedure for the power button is the same on both.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/samsung-4/samsung-galaxy-a12-nacho-1361/sensor-huella-y-bot-n-de-encendido-12290|Manuals / Samsung Galaxy A12 Nacho / Fingerprint sensor and power button - Nobody Calls Me Chicken|new_window=true]
There's a video that does match your phone exactly that might be worthwhile for you to view, as it talks about fixing the button that's not working right; as far as I can tell the button basically fell off it's mount and just needed to be remounted in order to work.
[link|https://www.youtube.com/watch?v=XXnqSUCVyOA|how to repair Samsung Galaxy A12 power on button #SamsunggalaxyA12 100%easy idq1009.official - YouTube|new_window=true]
Hopefully at least one of the things I've mentioned will help you get your power button going again.