I had a PC monitor do the same thing -- apparently one of the control rings on the tube broke loose. So for mine, if I smacked it hard it'd go back to normal. For a while.
I eventually ditched it, since the repair bill would have been higher than the cost of a new monitor. This may not be the case for you, the place I went to (which I trust) said that they don't do monitor work and it'd have to be sent to Seattle (~90 miles away at the time). The shipping charges to Seattle for a monitor are not cheap.
By the way. If you get it into your head to work on it yourself. Do not do so unless you know what you are doing! The tube works like a huge capacitor and can give you a very nasty shock.