The early E36's had problems with the electric windows, from personal experience it seems that the control arms pop out of the sliders and the window falls forward into the door - like this...............

 Take off the door card and you'll see one of the clips that hold the control arm has popped out causing the entire glass to fall forwards, from experience once it has done this once it will keep on doing it, so it's best to replace the white slider.

  When the glass falls forwards it is likely to fall out of the guide channel as well so you need to check this before you re-attach the clip.

 

Once you have made sure the glass is sitting correctly in the guide push the "ball" of the control arm into the "socket" of the new white plastic clip. Next is the hard bit which is keeping the ball pressed into the socket whilst sliding the retaining clip into place to secure it, this is what it should look like

 

Another view, here you can see the clip is sitting in the groove in the white plastic slider and is secured correctly.

 Next grease the channel the slider sits in and check that the window opens and closes correctly, if it pulls away from the edge of the glass channel you need to change the angle by loosening the bolt holding the adjuster - pushing it up makes it closer to the opening edge of the door, pushing it down moves it away from the opening edge of the door.