Sounds like your drive might have gone up to that big magnetic-media-playground in the sky.
You might want to read what
I wrote in this recent "Delayed Write Failure" posting over here. Also, is your computer's BIOS detecting the drive? If so,
and you can hear it audibly "clicking" (more than the normal drive-access "groans" you hear during boot) you've likely got bad drive bearings (or something similar). Either way, the drive probably pretty well toasted. There is a possibility that it could just be your IDE cable though (while that often manifests as failure to detect the drive in the BIOS, that's not always the case)... Always worth trying swapping out the cable before declaring the drive dead!
As far as the C: D: G: drive lettering stuff. If it was working before, and then stopped, and you weren't mucking about installing/removing drivers, playing in the registry, or deleting system files willy-nilly (installing or uninstalling serious software) then all of that is unlikely to be the cause. It's not impossible, but software
rarely "suddenly" causes the system to stop booting up anymore all on it's own (not counting Windows ME, that is).
At exactly what phase does it not boot? Do you make it through POST? Does the little white bar go across the bottom of the screen (the one that goes across just before the splash appears)? Does the Windows XP splash screen appear? Does it give you any kind of error or does it just freeze (or does it reboot endlessly in a cycle)?
EDIT: I just re-read one of your posts. You say it isn't physically detecting the drive. I'll assume you mean the BIOS doesn't see it. In this case, replace the cable completely. Try it again. If you're lucky you just got a crimp in your cable.
Also, I'm not clear if the drive that died is the one on the add-in IDE card but if so... I'm assuming you have this installed on an add-in card because your computer can't see the full size of the drive with your onboard IDE port. If so, try it on the onboard IDE anyway, just to see if the BIOS will see it (it won't boot because it won't see the full partition, but if it detects it you might be looking at a bad add-in IDE card).
Last, but not least, see if you can take the drive to a local computer shop and have them test it. If they can get it to spin up, it could be a more serious problem on your motherboard. This is fairly unlikely, but it would stink to buy a new drive only to discover your old one was fine and the rest of the computer was shot! (Lightning can do terrible things after all, and the storm can sometimes be a LONG way away).