Canon MX720 Series Online Manual page 201

6001
Cause
Printer error has occurred.
Action
If the paper output tray is closed, open it.
Turn off the machine, and unplug the power cord of the machine from the power supply.
Plug the machine back in and turn the machine back on.
If the problem is not resolved, contact the service center.
Important
Do not close the paper output tray while the machine is in operation; otherwise, an error will occur.
If you disconnect the power plug, all the faxes stored in the machine's memory are deleted.