Not sure how it would get that way... maybe it was never perfect from the factory? Maybe the yoke screw clamp has loosened after 20 years?
I'm not sure that you can really adjust the physical tilt of the CRT. It's pretty much screwed in place.
This sounds like a tilted screen image, which is corrected by adjusting the yoke by hand on the back of the CRT. If you look at the PDF I posted above it explains how to do this on pages 108-112.
Pages 97-106 explain what each of the pots does on that back of the Classic and none of them includes tilt.