I believe this is related to the use of a no-DVD crack (though I thought it was level 15 it did that on). Try playing with the original executable and the DVD in a drive; if you were already, and have more than one optical drive, try the lowest drive letter one.
I don't know where the saves are, but answered in your other topic. Someone with the game will hopefully see it soon and give a more exact answer.
You may need to revert to an older save (before you reached level 15), though I'm not sure when or what else the copy protection can cause problems with if it doesn't detect the DVD.
DON'T use no-dvd crack on this game, or else like 3000+ skill points, no more level up after 20?, load crash, no reponse from using teleporters, and many other weird bugs would be experienced...
They are not bugs, though; they are part of the copy protection system. Unfortunately, they look like bugs, and if people don't specifically look for a solution or patch, they may assume the game itself is at fault.
