Before you panic I'd check cabling - SATA cables aren't exactly tight fitting so can work lose over time. Something like a power glitch can cause this sort of thing as well.
I'd run a utility to check the SMART status of the drive - something like SpeedFan or HD Tune (free version). This will give some idea if the drive has problems - run an extended rather than a short test.
Treat the drive as untrustworthy and if you continue to get problems replace it.