Q: Unable to send or receive iMessages

Starting yesterday evening, my iPod Touch (5th generation) stopped sending or receiving messages. Every time I tried to send a message in an existing thread, the red exclamation mark with the message "Not Delivered" appeared. I tried deleting the threads and starting again, but now every single contact in my iPod is apparently "not registered with iMessage" when I know for a fact they are.

I've tried turning it off and turning it back on, resetting it, signing off and back on, checking for an update, and pretty much every supposed solution on these threads besides a factory reset.

Does someone know what's happening? Or how to fix it other than what every other thread has suggested?

Thank you.