The First Question Of Troubleshooting: what changed between the time
things worked and the time they didn't?
In the meantime, here are some other things to try:
1. Check the cable that connects the printer to whatever it is connected
to (the computer or a router if networked). Are both ends snugly in the
connectors?
2. If both ends are snugly in the connectors, try a different
known-working cable.
3. Have you tried turning the printer off and then on?