Why does my phone have to reboot after locking?

If I let my phone stay locked without using it for more than a few minutes, I have to reboot it to use it again. I don't have to reboot if I lock it and then unlock it again within a couple minutes.

Context: I had to do an emergency reboot after it wasn't letting me make calls, texts, connect to internet, etc, and I couldn't do a normal reboot. Now it functions just fine except for this. It also takes a long time to power up. Never had this problem before. Nothing out of the ordinary happened before this (e.g. didn't drop it or spill anything on it). Help!