check power and usb connections ok?
remeber if you bumped/dropped it etc?
can you hear it spinning up/powering up?
any wierd noises? grinding, clicking etc..
try on a different pc
if your on winxp, goto [my computer (right click)>manage>storage devices] see if your drive comes up there
if it is truly a machanical faliure, then you'd ask yourself how important is your data...
and like the painfull and expensive lesson i've just learnt, backup regularly.....