First clean the lens with alcohol as solvent - this is the most common problem.
Second, open up (this destroys any warranty) and clean out any dust, making sure the laser unit can move up and down on its screw smoothly.
Third, if the only problem is disk read error, disk spins smoothly, laser unit is moving up and down its screw, and other functions are working: a replacement laser unit (available on the internet for around £10) might get you running again. The ribbon to the laser unit has a tiny clamp which needs to be pulled up. The other two ribbons just pull out of the main board. To get the laser unit off the bar, it is easiest to cut off the plastic buffer and slide out the bar. Otherwise you have to unscrew the brackets holding the bar in place which have tiny screws and are very tight. You will need a soldering iron to remove protective solder and get the new unit functional (instructions should be included).
David
March 2008