Can't change rear gear.

[IanWright83]

Unfortunately today there was quite a bit of snow cycling home from work and during the journey my rear derailleur seems to have stopped working. Currently it's stuck in 3rd (out of 7) and I can't get it to shift up or down.

Initially trying to change up, the shifters would click but do nothing. Changing back down there would be no tension until I got back down to 3rd. After trying this a little, now when I try and change, the shifter clicks into 4th then does nothing, while changing down there is too much resistance on the shifter for me to want to force it. Unfortunately I don't know enough about bikes to fix it myself so was hoping for a few pointers as I couldn't see anything obviously wrong.

My bike's a Claud Butler Urban 100, the gears (I'm assuming the spec hasn't changed as these are off the website) are :

Shimano TX30 21 speed with STI shifters.

Any help is muchly appreciated! Thanks

[DaveM]

If this is the first time the temperature's dropped below freezing, you could have ice inside the cable housing. Try warming up the bike indoors or with a hair dryer/hot water to see if it fixes it. Otherwise, check for anything physically jammed in the cable or derailleur, especially under the bike.

[IanWright83]

Dave, as far as I can tell there aren't any jams and I think the cable is fairly well defrosted now. Think I'll pop it back to the bike shop and get someone who knows what they're doing to sort it out this time.