jessiljohnson, first off be careful. CRT display pack a punch and unless you know what you are doing, I strongly suggest that you do not poke or probe inside your TV. Take a look at the answers of How dangerous is it to work on a CRT display?
You did not tell us if it collapse horizontally or vertically. As a very general answer the fault could be with the vertical deflection circuitry. This could be caused by things like dry solder joints, bad electrolytic capacitors, vertical output I.C., vertical Oscillator and many more issues. A vertical collapse can be caused by the same things, but also by the Flyback Transformer.