Sounds like a component failure on the analog board so the horizontal sweep doesn't work. Larry Pina discusses this in The Dead Mac Scrolls. May just need resoldering of a connection, and/or replacing a capacitor or inductor. You should be able to find the actuall part references with google - I'm at work so I can't look it up in the book.

(A quick Google search finds this page: http://www.jagshouse.com/vertical.html - but I can't vouch for it without crosschecking with the Pina book)