Hello and welcome to Techspot.
In your monitor controls, does it have a degauss option?
If so use it.
If not, power off the monitor using the monitors power switch and unplug the power cable. Leave it off for a while, overnight would be better.
Plug it back up and turn on the power, this should run the monitors degauss coils. And don`t ever put a magnet near the monitor again.
Let us know how you get on.
Regards Howard :wave: :wave: