On August 13, 1961 the East German government built the Berlin Wall, thus closing off West Berlin from East Germany. Only on November 9, 1989 the wall was opened.
Although West Berlin was formally part of West Germany, it was not a separate Bundesland.
On October 3, 1990 West Germany and East Germany were united, thus formally ending the existence of West Berlin.
see also Ich bin ein Berliner