DVD-ROM Drive Not Responding

I have and old, ugly e-machine (633 Mhz, 128 Mb RAM) It came with a 12X DVD-ROM and I later installed a CD-RW drive. Everything worked fine for quite a while, then one day my DVD drive stopped reading anything you put into it. It still shows up in my computer, diagnostics say its running fine, but if you put in a CD it flashes a few times then stops and doesnt read the CD. I have no idea where to begin. Any ideas would help.