Bicycle Tutor

So I developed a problem with my rear shifter on this mornings ride to work and it's beyond my limited repair knowledge to fix.

I've got a shimano rapid fire shifter. I was running through the gears and the shifter was claiming that the chain was changing gears. The chain is stuck in first gear and won't move. I move the chain manually and same results. The derailleur moves perfectly fine, but the rear tire is moving very stiffly.

Any ideas?

If the rapid fire shifter is moving but not the derailleur then I would suspect the cable to the derailleur is broken.

See Alex's video:- how to change a gear cable.
http://bicycletutor.com/inner-shift-cable/