IceWM captured my computer!

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http://forums.opensuse.org – IceWM installed y default in OpenSUSE. I just decided to try it, but unexpectedly it became the default window manager (something new in behavior, 4 months ago the system asked before changing the default WM). Now I cannot logoff from it so I could choose another WM: after logoff the monitor says 'frequency out of range' (I wonder, why). Please give me an advise how me to return to Gnome. Currently writing from IceWM. (Distributions)