I had this happen to my unit. Remove all of the CDs from the caddy, insert the caddy back into the player and turn it on. See if it errors out. When mine errored it was because one of the CDs didn't eject itself back into the caddy, but after putting the blank caddy in there all was fine. She can also clean the unit using one of the disc based cleaners. Another thing to try is to be sure to insert all commercial burned CDs into the caddy...when I first bought the car a CD I burned myself caused the unit to error out, it seems that if there is one bad CD in the stack it won't want to play.
If worse comes to worse can she also get in the boot and unplug the power from it and reconnect it.