Did you try a battery pull? Maybe rebooting it will take care of the problem. OR, and this happened to me a couple times, if you let the battery charge get low enough, the cellular radio shuts off and you'll have to charge it a while to get it to come back on.
My screen went black. what do I do? I have no water damage, i din't drop it. what's wrong????
Usually a pull the battery will fix it. Probably ran out of memory.