Start the computer in safe mode, go to the display properties, reduce display resolution to minimum. Apply and reboot into normal mode.
Then check to see if you Display adapter and Monitor are identified correctly, go to the advanced tab in the display properties go to list all modes for the adapter and choose the on that is the native for your Monitor.
Then go to the Monitor tab in the same window and if the monitor is identified correctly, make sure the "Hide modes that this monitor cannot display" box is checked and on. choose the least of the displayed frequencies.... if there is just one choose that one.
Apply and reboot
I hope you haven't changed anything in the Catalyst center, if so, first get it all back to default and then do the changes in the windows display setting....