Well, could be a couple of things. The most obvious would be to degauss the monitor. If you have speakers sitting next to it, move them. If that doesn't fix the problem you may have a bad fly-back. You'd be better off replacing the monitor if that's the case.
I would recommend Team Excess for that. www.teamexcess.com
I bought my 21" Hitachi there for $230 plus shipping. Great prices.