I recently swapped PSU's in my system and when windows rebooted my hard drive suddenly became inaccessible.
The hard drive appears as offline in Disk Management, the disk is clearly working and connected as is shown in bios. Nothing about the system changed apart from the PSU. When i was having PSU issues i reinstalled a set of motherboard drivers, could this be the issue? I have tried right click>reactivate disk and it provides an error askin me to check the system event log, which just tells you to run checkdisk on the drive.
Try a different power cable to the HD. Are you sure the HD is spinning? Run Chkdsk /F.
If the power cable isn't the issue, perhaps you could try installing the hard drive on another computer and seeing if it's detected ok. Another option would be to boot from a Linux Live CD to see if it's detected.