First of all try to reset your unit.Unplug all the wires connected to it,and leave the unit for about 30mins.After that again connect the wires and start up the unit.
If it dose not help then,
The drive is not reading the disk properly. This is either caused by a fault in the laser/lens/drive itself which is leaving the laser misaligned and unable to read the disk or it is caused by dirt in the drive effecting the lens.
There is a Laser cleaning CD or DVD that you can purchase from any audio shop - its not expensive and useful to have around anyway. I would recommend you try a good qulity one such as this from Maxell http://www.amazon.co.uk/Maxell-Europe-Ltd-CD340-Cleaner/dp/B000001OM5
You may find that this will resolve the problem as a simple and quick fix.
If the cleaning disk does not work then you have to go mannualy,
To do so, remove the cover of the device that's you wish to clean, once you have done that, you may be able to locate the laser eye without any more dies-assembly, if not, dies-assemble the laser eye housing or what ever it is that's restricting access, please make sure to note, either with a diagram or marking with a coloured marker what screw came from where and how to re-assemble the unit.
Once you have done that, simply clean the laser with a soft clean cue tip or a cotton bud, the style used to clean ears with. After wiping , check with a magnifying glass for any traces of dirt or grime.
Once satisfied with its appearance, put it all back together and play some disc.
It will solve your problem,if not then the unit will have to go for service repair .Either the lens or the driver has to be replaced.As the unit has been running for a long period so the lens has weaken.By completely replacing with a new one will get your unit going once again.
Maxell Cd Lens Cleaner