Unplug the system, disconnect or remove the battery and remove and reinstall the drive.
Plug back in and see if the drive appears. If it does, back it up -- NOW.
If it still doesn't show up, it's too late for backup. If you need the data, you'll need a data recovery service such as Gillware, Ontrack, etc. Contact one of these services for an estimate (but budget $600 or so for a basic recovery and depending on what needs to be done, possibly into the four-digit range in dollars if it's an extensively damaged drive).