What could be the problem with my window?

Ok I have an 03 Ford Mustang. My driver side window was working fine but the passenger window wouldn't go down. Then one day I tried to let the driver side window down it wouldn't move no buz nor did the window look like it was trying to move. What's crazy is the passenger side started working. Can someone help me with this? Just in case if I have to go to the shop about how much would this cost to fix?

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