Hi there David Strutton,
Most probably, there is not a particular cause for this. Drives(being made of mechanical parts), just fail. You just need to store your data on at least two places.
Does the computer boot properly? In case it does, it may be a good idea to back up the data as soon as possible. In case it does not, you can try accessing it with some sort of data recovery tool for DOS.
Cheers,
D_Know_WD