Rear Derailleur Won't budge

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I'm sure its a fairly simple problem, maybe one of you can give me some tips. I was taking a few jumps with some buddies of mine on a ramp we built in the backyard. Everything on the bike was working beautifuly until i noticed that i was unable to change any of my rear gears. I noticed, as i had one of my buddies hold the bike up while i rotated the pedals, that as i shifted slack either built up to hang below the bike or dissapated in the cable on the undercarriage of the bike but the derailleur did not move at all. I'm stuck forever in 9th gear! I'd rather learn how to fix the problem then rely on a shop to do it for me, any ideas?

I'm sure its a fairly simple problem, maybe one of you can give me some tips. I was taking a few jumps with some buddies of mine on a ramp we built in the backyard. Everything on the bike was working beautifuly until i noticed that i was unable to change any of my rear gears. I noticed, as i had one of my buddies hold the bike up while i rotated the pedals, that as i shifted slack either built up to hang below the bike or dissapated in the cable on the undercarriage of the bike but the derailleur did not move at all. I'm stuck forever in 9th gear! I'd rather learn how to fix the problem then rely on a shop to do it for me, any ideas?Try detaching the rear derailleur cable from the rear derailleur, and ensuring that everything moves freely MANUALLY ...

* the derailleur -- right-to-left-to-right (probably easier if the rear wheel is out of the frame)
* the cable inside the housing

DIRT is probably the problem, and cleaning & lubing may remedy your problem.