Mine did the same, except it still played CD's and "home burned" DVD's (photo discs, etc...). Tried cleaning, but no good. Once I opened the case I nudged the main laser assembly forward. Let me emphasize 'nudged', be gentle. The assemly slightly slid forward with little resistance. This seemed to do the trick. It realigned and plays fine again! This will probably only work if it still reads some discs, otherwise your option would be replace the unit, yuck!
Ted
February 2008