Generally this can be cured by carefully cleaning the lens with a Q-tip with alcohol on it.
Another concern is that the CD motor spindle may have gotten pushed down a bit by putting in CDs too firm. This can cause the focus to be so far off it won't work.
It's a fairly precise measurement, but if there is more than a whisker of the shaft sticking up from the middle of the spindle where you puit the CD, it may need to be pulled up a little tiny bit.
I don't mind giving this advice to tinker with it because CD players aren't usually worth having professionally serviced any more.
William Miller † MicrowaveDisplay.com, March 2007
