To fix this, you need to do something called degaussing. You should keep magnets away from the screen in the future, because they cause this problem.
The first thing to try is to turn it on and off, making sure that all magnets and speakers (which have magnets in them) are kept far away from it. This will activate the monitor's auto-degausser if it has one.
If this does not help, this guide: http://www.wikihow.com/Degauss-a-Computer-Monitor
gives a couple solutions, including using a soldering gun (not a soldering iron
), illustrated in the video below:
With a powerdrill:
Or investing about 60 bucks in a degaussing coil like this
which is a guaranteed fix:
One other trick that might work is to take an old CRT monitor ("fat-screen" rather than "flat-screen"), hold it up to the tv screen to screen, and turn it on and off to have the monitor use its own degausser. The monitor may have a button to activate degaussing, or it may be in the menu you can bring up with the buttons on the bottom of the screen.
If you don't want to deal with any of this, a tv repair shop will degauss your tv for you quickly.