The usual cause is a pair of unshielded speakers, but one of my staff's grandfather had brought his Max/Min thermometer, in from the greenhouse and put in on top of the monitor. This has a small magnet to reset the bars and was causing the screen to deform.
on your monitor you may have buttons to enter the monitor menu system. If so find the Deauss option and select it. This may cure the problem.
If this does not work try and find out what software has been installed recently, some games or screen savers may change you monitor settings. If this is the case right click anywhere on an empty space on the screen and select properties-setting-advanced, this will allow you to alter the screen settings. Some offer the settings to be set at default which is a good place to start.
If no joy there its worth checking the driver for the graphics card and updating it, also check the plugs and possibly the card is seated properly in the case.