If a CD player doesn't work there isn't much the user can do - putting in a disc the mechanism should be heard to spin; if it doesn't spin the process will go no further and it is a task for the repair shop.

If the disc does spin the laser will try to read the disc and if it fails it will be worth cleaning the lens but again if that doesn't work the repair shop beckons.
I have never seen the DEF message and don't know what it means unless it is an abbreviation of defective...