If it's the same drive that it originally shipped with then it's probably just died a death (it is/was nearly 20 years old) - the Quantum's that Apple used don't seem to have a very long life at all (when I got my hands on a Mac II the HD was just screwed and had to be binned)

Turning it over won't have killed it. OK it might have nudged it over the edge but HDs will happily run upside down

Get excited again - you now have the fun of fitting a new drive and starting from scratch