MAKE BACKUPS OF YOUR SCORE OR CAMPAIGN FILES BEFORE TRYING THE FOLLOWING:
The game must be able to detect whether or not you still have the original disc in the drive.. as long as you are not trying to use a duplicate of the original store purchased cdrom disk, then..
Your problem could be caused by several other things similar to old age or careless cdrom disk care.. one of the two following possibilities needs to be checked:
1) The (favorite pet) chewed on your cd.. or you are in the habit of tossing cdrom disks on your table while switching to other games, and the disks are not carefully stored away from dust, dirt, heat, moisture etc.. and with various scratches appearing, your cdrom disk is no longer in pristine condition.. causing some data loss due to certain important sectors on the cdrom being unable to reflect the lazer light properly (try another copy of the game from the store or a friend's original disk of same version you already own in your computer, or uninstall and reinstall a different version)
2) Your cd rom drive mechanism is very old and is starting to lose its ability to stay aligned with most other cd disk games.. (try other disks from other games to see if your drive still reads all of them or not, or try your disk in a different cdrom drive, or different computer altogether)
The game may or may not be able to be re-installed but you could try.. during reinstallation of course the entire cdrom disk would have to be read into the computer again, so this may not be possible..(make sure you backup your character and campaign files just in case you want to save your scores).
What NOW!!?