Best option : Get it serviced from an expert.
Risky option :
I had a similar experience with my Nikon D3100. The camera fell slightly. The zoom was stuck about mid-way. I could not zoom any further. After having a heart ache, I held the lens and slowly turned, did not work. Then held tightly and it worked. I dont know what had happened, but things came back to normal. I have been using it in that condition for past 6 months and am careful not to do things v fast. Zoom works good, focus is good.
I think, in my case, the issue is, the fall broke one of the gear teeth and when I give it a little push, the gear continues its motion.