Could be a number of things.
Does it still load the disc into the machine and does it still spin?
If it still loads the disc then the servo mechanism is okay, if it still spins then the drive motor is working okay and it’s possibly the laser unit.
I wouldn’t recommend this under normal circumstances but if that’s the case then you’ve pretty much got nothing to lose so try cleaning the lens and see what happens.
Unless you’re going for more up market equipment in general the DACs in cheap DVD players and cheap CD players are pretty much comparable these days.
If you’re connecting it to an A/V receiver or surround sound system though you’ll be able to hook up the DVD player via a digital connection and that should improve the quality to some extent.
I’m not sure if modern dedicated CD players now have this capability but you’ll definitely be able to connect up the DVD player this way.
Or you could get a cheap Blu-ray player instead.