The chances are that it has been marked as 'inactive' so the operating system is ignoring it. Happened to me a couple of times.
If you go into the disk management system you should be able to resurrect it or you could try the 'add new hardware' route which should send the os off to look for it.
Otherwise run a partition manager and it will display all the drives on the machine and allow you to change the drive status.
I assume. of course. that you have checked that the data and power cables are still connected.
Last edited by A A Gruntpuddock; 7th April 2011 at 00:40.