I used a heavy plumber wrench to give the button several whacks and lobehold it sprung loose and the square ​plate shot to top position.  Now it is movable but would not stay in middle range, so either all the way top or stuck at lowest position, which is better than before. Button was loose when sq plate is at top, but stuck again if plate pushed down to lowest level. It is like spring loaded and cocked at bottom level.  Guess it is degenerative.  Still has life left. 
I got three stands that have the button stuck.  can not raise the monitor.  not sure if there is a fix. [IMAGE] [IMAGE]  Dell LCD Monitor Y-Base Stand Tilt Swivel Rotate for 1907FP 1908FP
dell 19" LCD monitor stand adjustable button stuck