HJ - be careful, it is quite possible to blow up the monitor (I have witnessed it!) and even start a fire if the monitor is overdriven.
What is the monitor, and how old?
You should attempt to find the monitor in the list of available monitors from Display -> Settings (Tab) -> Advanced (Button) -> Monitor (Tab), then ensure that you check the box that states "Hide modes that this monitor cannot display".
Note that you often have to trade-off Colour Depth / Resolution / Refresh Rate, depending on what is most important - it is often possible to select 1024x768 and 16-bit colour, but only at 60 Hz (hard on the eyes), while the same monitor could display 800x600 and 16-bit colour at 72Hz. Personally I always go for highest refresh rate for a minimum 800x600 and accept whatever colour - even only 256!
Fortunately, monitors have mostly moved on!