Tyre Kicker,
I'm assuming it is the optical drive you're talking about here - pretty sure from what you say but want to be absolutely certain.
Is the drive one of those that you can eject out of the laptop & switch for a floppy drive - I had one like that in an IBM Thinkpad. If it is I would suggest you try reseating it by ejecting it & putting it back in securely.
Next step for me would be to look at the BIOS settings and make sure that the BIOS is recognising the drive. There are usually help screens to give you guidance, its a matter of catching the laptop as it first boots to get to the BIOS.
After that it starts to get serious. If the BIOS doesn't recognise the device try adding it, but things like that don't normally change on their own & I would be starting to think about getting it professionally checked.
If however the drive is showing in the BIOS then I would let the laptop boot. Again I'm assuming here that it is Windows XP you're running, and I would go to control panel, and then look under Hardware to see if the drive is accepted by the device manager. The Microsoft web site will give you some clues there.
Good luck with it - let us know how you get on