If you are loading regular audio cd's (not mp3 or burned cds) and it gives you a "bad cd" message then either the player itself is dirty / the laser pickup is dirty or the disc itself is not spinning (as a result of a faulty or failing spindle drive mechanism.
When you insert the cd, can you hear if the cd is loading or spinning? If it does, then at least the motor/spindle assembly should be ok - but the pickup could still be dirty or smudged
Unfortunately, the only way to properly clean the laser/lens is to take the thing apart. Do not use one of those DVD/CD cleaners with a brush on the underside - those are not meant for slot-loading players.
If after cleaning and it still does the same thing, then it might be time for a new aftermarket deck or an oem replacement deck
goodluck