My monitor has gone from bright to very dull... Anybody know why?

Try recalibrating your screen.

System Preferences>Display>Color

then click on "calibrate", go through the guided process of recalibrating your screen and you should not be experiencing any more washed out colors. If you do then you might want to have your screen examined at a store.

just to add, option-click Calibrate, to get more ca;vibration options.

Turn on expert mode, and follow instructions