The disk is still there. It needs to be reactivated.
Follow these instructions (For Windows XP):
Go to "Control Panel"
Left Single Click on "Performance and Maintenance"
Left Single Click on "Administrative Tools"
Left Double Click on "Computer Management"
Left Double Click on "Storage"
Left Double Click on "Disk Management"
In there you will see a list of Disks - typically labeled "Disk 0", "Disk 1", "Disk 2","Disk 3", etc...
Right Single Click on the Disk that is not working or not activated (make sure that you right click in the area where it says "Disk(n)") then
Left Single Click "Reactivate Disk."
You might need to do some maintenance (check disk) on the disk after it is activated.
There might be an easier way to get to your disk management if you have "Administrative Tools" activated on your start menu.