Since the host drive will always be seen as C even if you have more then one drive used for multiple OSs the other drives on the system may suddenly become invisible in the MyComputer or Windows Explorer drive listings there. Those will still be found when first starting the system on the post screen if board manufacturer's logo is disabled and in the Disk Management tool found in "Control Panel>Adminitrative Tools>Computer Management>Storage".
If the drive or drives are not seen during the bios post test or in the DM you may have to either replace a cable or drive if it failed? The more likely cause is that a Windows driver will need to be reinstalled for proper detection of the one or more drives involved. The usual method that sees quick results is to start off by right clicking on the MyComputer icon followed by clicking on properties. Then open the "disk drives" section and right click on the effected drives in order to choose the uninstall option seen in the right click menu. From there restart the system and look for the "new hardward found" popup message seen on the taskbar.
Simply right click on the hardware icon and have Windows perform an automatic search for the needed drivers. Windows already has them to install. For XP Home edition you may have to restart the system again to have Windows fully reinstall them and have the drives then reappear.