Your lens may have been dropped. Since AF is okay, one possible cause of the manual focus not working is the off position of the silver face cam (retaining ring) i.e. the top ring of the USM assembly. It may be as easy as a slight twist to tighten it up and to bring the manual focus back to life. Or you may have to do several adjustments to balance the right tension for both MF/AF. There may be other causes as well.
Another post here in this forum discribed how I repaired a L lens with the similar USM unit.
http://community.usa.canon.com/t5/Lenses/Canon-EF-24-70-F2-8-manual-focus-slip/m-p/165429#U165429
Do not attempt to open up the lens to repair unless you have some experience doing so or you won't mind sending it to Canon for repair and pay something like $300 or so to make it right. Flex cables are easily broken and it is not easy to dismantle and put things back properly. DIY repair is at your own risk.
Good luck.
