cavortingcheetah.....
I guess it could be a variety of things, but on your TWAIN line of thought, try interrogating it from an external program and see if it is recognised/working. Most photo software, Photo Shop, Paint Shop Pro or similar, will do the trick via File > Import (on some programs a further menu: > Select) detecting the scanner (or not!).
Any drivers on the Mustek website you could try??