I have a windows 7 64 bit machine and the RAID drive recently disappeared from the Computer window. It is still visible in eDOS and in Disk Management... -scowl-
First I would check that it still has a drive letter. In disk management, right-click the drive and go to "Change drive letter and paths". If it already has a letter, try forcing a change to a different letter.
If the file system on the array is not recognized you may have a more serious (hardware related) issue at hand.