Re: E90 PFL power window
Common issue. Happened to my BOTH rear left and right side - just 2weeks apart! Most likely the window lifter assembly.
Try pressing on the power window buttons and listen for the motor sound. If you can hear the motors working, there's a good chance that they are still good.
Also, if the lifter is broken, u may be able to use your hands to physically pull up the window. Do that and place a suction cup at the bottom of the window to prevent it from falling back down as a stop-gap measure. With the rain these days, not advisable to drive around with the windows down.
I bought my window lifter replacements from the nice people at Eng Soon, and got them replaced at my regular workshop. Should be a small issue.