First port of call for all electrical or microprosessor controlled systems is to disconnect power to the unit then reconnect.
Logic circuits do get confused from time to time so a reset usually sorts these out, either pull the power supply out from the rear of the unit, or, disconnect battery for a few seconds, once reconnected all should be rectified.
If this still is not fixed then a good clean of the laser is the next job, cleaning disks dont work - laser tracks a few microns above the surface of a disk so these "cleaning" disks dont do anything at all, Dust can settle on the laser head which in turn deflects the light causing a mis read, a simple cotton swab used dry rubbed over the laser head will sort that out, however, you will need to pull the unit out and open it up to get at the laser, and if you do this then you may as well polish the laser guide rails with the swab just in case dirt has caused the laser guides to stick, you can move the laser head manually by just pushing it along the rails and cleaning the exposed part of the rails, as you reconnect power the laser head will auto reset itself into position
