If you can roll it off, most likely it is in the throtle control in the handlebar end. Take the assyembly apart and wipe it down (no lube- attracts dirt that turns to sticky goop). As a back up, check the linkage at the carbs to make sure nothing made a winter home in there. It's a simple setup of a cable, linkage and spring. If there are no obstructions at the linkage and the throttle control isn't gooed up, the cable may have gone bad. If you try to lube the cable, use a dry lube like graphite powder.
How old is it?
