what cable is used for the connection to your PC and have you tried a different one already?
Has the monitor worked with the desired resolution in the past?
Can you test the Monitor with a different PC/Notebook?
Can you test a different Monitor/TV with a resolution higher than 1200x800?
Have you tried to load the monitors factory defaults?
Have you installed the latest drivers for your GPU?
Please note that this is a German community forum, which is moderated by German speaking tech support. If you are located outside of Germany, Austria and Switzerland, I'd recommend you to use the English community forum instead.
With kind regards,
Florian Steinau Community Technologist - Business Client Social Media and Communities Professional Twitter | LinkedIn | XING