slipped c clip (b tension adjustment).

[bikingdad90]

I have a rear derailleur where the c clip has slipped and all the tension has unwound so that the top jockey wheel now touches the cassette.

The screw is floating and is on the wrong side of the mech hanger. Is it possible for a bike shop to reset the mech, use it without or is a write off?

 

[Smurfy]

You can get the tension back yourself:
1. Remove the screw
2. Hold the mech body in one hand, and turn the tab that the screw goes through with pliers
3. Lodge the stop behind the appropriate raised part on the mech body
4. Replace the C clip

I've done it myself plenty of times when cleaning. Unless you have good manual dexterity, you may need a helper to place the C clip while you hold it all together. Wear eye protection in case it pings apart in your face while you are replacing the C clip.

http://www.parktool.com/blog/repair-help/rear-derailleur-overhaul

Edit: If the screw is merely on the wrong side of the hanger stop, you probably won't need to do any of the above. Just remove the mech and reposition correctly.