What is stuck? the shifter, the cable or the derailleur; and front or rear?
Lots of people on this site know how to repair bicycles, but none of us can with an adequate description of the problem, which usually requires pictures.
First - is there an rust/corrosion on the cables? If yes, the cables and housings need to be replaced.
Second thing, disconnect the cable from the offending derailleur. Can you move the derailleur by hand across its range of travel - you will have to rotated the pedals while doing this or remove the chain?
Grab on to the end of the cable very tightly that you just disconnected, actuate the shifter, is cable pulled smoothly in and out?
Remove the housings, and repeat the above with just the cable coming out of the shifter.
If this is for the rear derailleur; make sure that the housing between the frame and the rear suspended fork is long enough. On bikes without suspension, the housing should be a short as practical, on bikes with rear suspension, there needs to be enough of a service loop to provide consistent performance over the range of suspension travel. On the NEXT this is particularly important.
