What is causing the trouble in my monitor and what should I do to solve it?

About two years ago, the screen of my monitor suddenly changed into blue. It seems like everything was tinted in blue. The computer, however, runs normally, except that some programs that require the use of colors can't be used because of the problem. By giving my monitor a few taps on it, the screen returns to normal, and then after a while, it goes back to blue again. But now, the problem has gone worse. My monitor changes into a very dark color. It causes pain in the eyes and most text are unreadable. What is causing this and what should I do? Changing the brightness / color won't do because they don't have much effect anymore.