Hi I have have a 2003 Cooper S and my power window on the driver's side stopped. Just trying to find out where to start, the switch or the window itself.
It was working fine this morning Thanks

 

Give the door a good "thwack" with your hand a little above and rear of the door speaker...it works in most cases until it quits again then you give it another thwack! I know its not a permanent fix but it'll keep you going until you need to replace the motor etc...

 

Mine's been in the "thwack" mode a couple/few times a month for a couple years now.
More often in the summer after it sits in the sun for a day or two.
I did shift the magnet in a Polk speaker in the door a year ago, probably from repeated thwacking.
I had to get a replacement mid/woofer driver.

 

FWIW I had a 1st generation Miata as my previous toy car and every time the windows acted up it turned out to be the switch. I replaced the switch the first time it happened; after that I just started disassembling the switch and cleaning the contacts to get them working again. Apparently, arcing inside the switches caused the contacts to get pitted and stop working.