This sounds to me like Windows has not detected that you have changed your screen and it is keeping a resolution that is higher than the maximum, your screen has a higher resolution than the motherboard supports or not all video modes are being sent by the monitor (or possibly more).
What I recommend you do is take a look at the monitor specifications and see what the maximum resolution is and if you do not see this as an option, go to advanced and tick the box "Enable all screen modes" (or similar), and then set it.
If it does not work, you may be out of luck, but try upgrading graphic card drivers.