Are there any alarms? Generally, when a controller boots up in maintenance mode automatically it means the memory has been wiped. This could be due to a dead battery in the controller. It could be a stuck Main Menu button on the pendant, assuming it has the standard pendant.
Go to SYSTEM then VERSION. In this screen you should see the application, like GENERAL, and the model of robot attached. If there isn’t anything list the memory has been wiped. Reload a cmos.bin file if you have one if not call Yaskawa.