Rear Derailleur wont shift

[SteveHammersley]

All of a sudden, the rear derailleur on my bike wont shift. I can move it manually by hand and the spring works fine. Adjusting the two screws makes no difference and the cable tension seems fine. On the handlebars when i try and turn the shifter it is solid as a rock, if i release the tension, it moves fine so I think all is ok at that end. Never seen this before and can't figure out what is wrong. All help appreciated.

[trevgbb]

You don't give makes or types, but I guess this is a rotary shifter, in my opinion, none of these work very well, but there is one around, usually fitted to bottom end bikes, which is almost impossible to move. Can't remember the make, but it has red lettering and gear positions.
Personally I would junk rotary shifters and fit almost any type of thumb shifter in preference.
But a new stainless wire and new outers may help, make sure that all bends are gradual and nicely curved.

[cyclerUK]

Check out the cable.
Undo the inner cable at the derailleur and pull it out from the outer casing.
Look for loose strands or rust. Pull it out right up to the handlebar end.
Check that the dearilleur will move the full span of the cassette.
Check the changer also moves through its full range.