A magnet should not affect an L|CD screen in any way, only old style CRT monitors (ones with a Cathode tube).
More likely it was dropped or had something dropped onto it.
If you can take the screen apart yourself make sure no cables have come loose.
If not then take it to a local repair shop who should be able to replace the screen for around £120-150 assuming it is a screen fault and not just the inverter that lights up the screen.
Also check that the machine has not been set to external monitor mode, usually Fn+f10 (or whatever button has a screen logo on it)