The windows in your BMW seal tightly into a window channel or weather strip, when closed. During the long hot summer, the windows are infrequently operated. Heat causes the rubber weather strip/channel to stick to the glass.
To raise or lower the window, a window regulator or cable and pulley system is employed. After months of non-use, excessive force is required to release the window glass from the weatherstrip. At the moment of initial restraint, the cable pulley/window regulator system can be damaged; thus, rendering the window inoperative.
To help avoid this problem make it a habit to operate all windows on a regular basis (weekly). In addition, all window weatherstrip/channels should be lubricated as prescribed during an Inspection I/II service.