I had this happen with a lens that I was given when I was young and foolish. It welded itself to the 80mm end of a 28-80 lens I think. One end, anyway.
My solution back then was to put the lens in a vise
(padding it so it didn't scratch it, but held it firm) and grab the zooming end with one of those things that opens lids of jars and twist. It budged! Apparently the lens had been hit and was slightly "out of round" so to speak.
I'm not recommending this, because my older self realizes that more could go wrong than right...but it did work back then, and I still have the lens - and the mechanism is still very stiff! So, if you're desperate...