virus???

I have a question out to anyone who’s experienced this. I’ve now had to instances of pc’s that I’ve troubleshooted, where the system is fine one min., and the next, it shuts down by itself, and then upon reboot it appears as if the h/d is completely blank. Has anyone else heard of this happening? If so, are there any troublehshooting tips to recovery of the data, or is it gone for good?