It is extremely likely that the disc is too dirty to play...it is surprising how grubby discs get and the fact that players are not as good at playing through grime is often not understood....make certain your discs are kept clean and free of fingerprints.
Also, a smokers or gas fire atmosphere can cloud the laser lens so cleaning is the only option here. Unfortunately, cleaning discs are either brutal or useless, there is no effective cleaning disc on the market that I have ever known to work well.
Also, give the disc a gentle few spins by hand first just in case the motor has stuck or has a dead spot.
CAREFUL OF MAINS AREA !!!
if all else fails....bin it......responsibly!
daveB, January 2009