im not sure but try checking all your cables are firmly connected and check the graphics card is firmly in the slot.
then check control panel/display/settings/advanced/monitor and tick the hide modes this monitor cannot display box,then select the highest available refresh rate and apply ok.
hit the degauss button on the monitor.
update your graphics card drivers ATI click here NVIDIA click here
could be a faulty monitor or graphics card if none of the above sort it.