Check if Windows still sees the drive first in Computer Management (assuming you're using XP) - right click My Computer > Manage > Device Manager and see if it's listed under DVD/CD-ROM drives, and without problems (the yellow ! or red X).

If that's fine, look in Disk Management on the left, and give it a drive letter in the list on the bottom right - if it's already got one, remove it and reassign, and see if that fixes it.