You will probably need to take the drive out of the computer, and disassemble the halves of the drive's case. Then, re-engage the tray into the rack-and-pinion gear that drives it.
AS DVD drives are relatively cheap (and ever-improving spec) you might be as well to just get one of the latest ones before you open the PC.
BTW if you haven't opened a PC before, the main hazard (apart from the obvious) is that some cases have razor-sharp metal edges!
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Try fixfault.com to fix desktop pcs.