I searched on the internet and saw a couple of cases that zoom ring get stuck after many years of usage of
the Canon 75-300mm USM lens.
There seems no solution to this.
Recently I had this lens and I opened it and did a DIY fix.
The root of the problem is the arm that connects the outer zoom ring with the inside zoom barrel gets loosed after many
years of usage. The DIY fix is to drill extra two tiny holes to add extra screws to fasten the arm into its location.
See the step-by-step video I had on the youtube:
Also add the key picture here:
Hope this will help someone has a similar issue to get it fixed by themselves.
Comments are welcome!
The best answer for the 75-300 is to get a 70-300 USM II. It is a budget level zoom anyway.
Good advice. It is low cost lens for a reason. If the new camera owner does not catch the photography bug, and the camera kit winds up in a box, in a closet somewhere, then the initial investment has not been that great.