Why don't u try asking in the canon thread see if anyone have an answer for you. https://forum.lowyat.net/topic/3293955/+260

I do not own a 28-70, but if you send to canon centre in peremba square they will check for you for free and give you a quote but be warn official service centre always very expensive.

I am imagining in my head what could be wrong inside, could you explain more, could be a screw stuck making it block the internal barrel movement, or misallignment after reassembly. I am unsure about the build of the lens but maybe u try loosen the zoom ring rubber see if any screws you can reach in the inner barrel and righten it. (Im not sure if there is even a reachable internal screw there, just giving you a hunch solution. try at your own risk)

Really need to open to see what is wrong