Sounds like a build up of dust on the lens.

2 options -

Either buy a DVD lens cleaner

Note DVD LENS cleaner, NOT DVD Cleaner the latter is for the discs, not the player

or second option is to remove lid from your player (usually 5 or 6 screws) and gently swab the lens with a cotton bud or similar soft lint free product. Try not to push too hard on the lens or you may damage the mechanism.

The lens is really obvious as it faces upwards and looks like a small round clear glass bead.

Make sure its unplugged too!