I set display contrast to maximum, slider disapeared, ican't get it back!

problem solved! I pressed tab key for several times till it could find and locate the slider, then pressed the left arrow to turn it down :D

From >System Preferences> Keyboard>Shortcuts>Accessibility> Here you can uncheck the Contrast shortcut keys.

This will keep from inadvertently from the key combinations from slowly but surely making your screen more whited out.