Undo the cable from the caliper, and check whether it releases then. If so, you can be sure it isn't the spring or pivots of the caliper.
In that case, as Marco says, it'll be the cable inners are f*****. Had the same problem with my Trek after it had been sitting doing nothing for months over the summer. If the cables are more than a year old, skip to the end of Marco's advice, just replace them!! Oh, and do it yourself, for the price of a service, you'll easily get yourself a decent pair of cable cutters. It's not difficult!!