It's very serious. Depending on the failure cause, the HD can be subject to overheating and this can cause operational problems. If you freeze it, it could get cold enough to give some operation time to recover some files at a time.
I've seen 2 hard drives recover from a death click with a good, opened handed smack - was never a permanent recovery, but good enough to offload the files. We used to call it a "percussion adjustment".
I've done this with a few drives that have had a head crash...works about 40% of the time....turning the drive upside down and giving it a good, solid smack on the top of it with the flat palm of your hand as level as you can works about 60% of the time.
Either way, if you can get it working at all...get your data off of it ASAFP.
Also, put it in another machine that does boot (make sure the drive is setup to be slaved off the boot drive) and hope like hell it'll at least init.
my friend was adding 2 drives to his 4.5 terabyte server with like 1500 movies and 100 full shows and the power went out........lost everything... not really related to this but still sucks ass nonetheless
Had one go out on me not long ago. Fired up the old Dell and got the drive running outside parameters warning. Backed up everything and two more boot ups the fucking thing didn't even show up in the system. It pays to constantly back up everything.