If the mechanism is siezed then some lubricant might help. Most of the mechanisms out there have plastic parts inside and the gears are easily stripped if forced so you need to take care. If you can't get the window down with the winder you may need to look at the pivot arm and disconnect it where it connects to the window frame, that will then allow you to close the window until you can fix the mechanism.
Hope this helps, regards Steve.
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Steve, Di & Ziggy We named our Motorhome "Roadworx" because on the road works "On The Road Again" Ford Transit with 302 Windsor V8 conversion, C4 Auto, 9 Inch Ford Diff All Lighting L.E.D., 260 Amp/h AGM, 530 Watt Solar + Kipor Backup Gen.
As madaboutled has already said, it is very easy to strip the plastic gears, especially if it is an older caravan, then you will have to disconnect the arm at the pivot
Further to madaboutled advice
Sometimes after spraying plenty of easy glide type lubricant, then winding the window outwards again until it is level, if someone on the outside puts just a little bit of pressure, on the side of the window which is stuck, it may come back in, while someone else is winding it inwards.
Hope this helps you out, and let us know how you get on
Thanks everyone for your assistance. We ended up pulling the plug out of the winder box and using both the winder on the right side and a makeshift winder on the left wr got the window closed will get it looked at when van goes in for next repair.