That is frame collapse, a very common fault.
It's usually quite straightforward for an engineer to repair.
Often caused by frame IC failure, dry joints or leaky electrolytic capacitors.
Expect a repair cost around $80-$120.
The repair should only be carried out by a qualified person.
(Don't operate the TV with the horizontal line showing as the intensity of the beam could burn a permanent image of the line onto the screen phosphor.)
Cobweb, January 2005