Could be a switch, motor or a wiring problem. Try jiggling the switch a few times, this is often a switch and doing this will get it to temporarily work. If that doesn't work, pull the door panel and diagnose with a voltmeter or test light.
Find door panel instructions here: http://bit.ly/mitchell_repair_manuals_online
My husband has had to fix both the driver's side as well as the passenger side widows since we bought this truck 4 weeks ago ----- he was able to pull off the door panels, make sure all the electronic connections were good, and when doing so it re-connected what was needed.