I heated food in my microwave on a paper plate and it started to smoke. I had a ton of smoke in my house but now it is cleared. Should I be worried about the microwave? Is it still safe to use?
Soot is electrically conductive, so if any got inside the oven internals, the oven might fail or react erratically at first. That brings a small risk of an electrical fire, so it would not be advisable to run that microwave unattended for a while.
The Problem with even plain paper is that we do not know what else apart from wood fibre might have been in it. On the other hand, Paper is not infrequently used to light barbecues.
Obviously, if you can clean the chamber and fan/fan duct thoroughly of all soot and residue you should be golden re: food safety; if you cannot access these easily ... if you couldn't tell me straight why pulling the plug alone will not always make the device safe, do not attempt disassembly yourself, please ask an electrician for assistance (if oven is expensive) or replace the oven (if not expensive).