Stuck up or down ?
Mine was stuck down and I found that pulling (hard) with my fingers on the glass whilst pushing the button got it to go up again.
The problem was a filthy window and window seal. I cleaned them really well and the problem went away.
The motor is probably undersized for the huge windows on the Multipla. Dry grit, dust and filth really puts a drag on the motor pulling the pane against the seal.
You might have burned the motor out ? Are the wires crimped in the door. Any whirring in the door panel when you push the button ?
Does your Multipla live indoors ? I found that my windows went up and down better on wet days outside when the glass was lubricated with rain.