DVD players, for the most part, are designed to be disposable - especially if it cost less than $50.00 USD. I bought a couple of el-cheapo players (one APEX, then other a COBY); spent around $40.00 USD on them, neither lasted more than a year or so. After that, spent about $80.00 on a Sony unit; haven't had a problem with it for a few years now (but if it does give up the ghost, I'll just buy another).
Highly unknown what or where your problem is; it could be any number of things. I would first try a cleaning disc (start with the stupid simple stuff first). I have also seen (in a CD player, though) where a speck of the "label side" of a disc (tiny little flake) got stuck on the lens of the read head; nothing helped (cleaning disk, qtip with alcohol, blowing) until I looked closely at the lens, and spotted it. No idea how it got there, but it must've flaked off a disc and lodged there somehow. Once it was removed, CDs played fine again (much to the relief of my wife, who likes to listen to books on CD).
Beyond that, it could be the laser, motor, focusing coils/actuators - any number of possibilities (and if it was a DRE on a Sony PS2, there are a few things you could try, but even there, you might just have to "give up" in the end - not worth the money, IMHO).