Damage to vehicle after Service

I have a 1800 utility Chevrolet. Its in mint condition. We took it in for an oil change and replacement of the clutch and skimming of the flywheel. It is so clear that a bad job was done, now my gearbox is giving issues, something was loose, and my gears is not properly working. i am being treated like i am not a paying customer. And no admission of fault from their side. How can you install a new clutch, and something is loose and the gears cant go in properly. i am sorry, but this, is not right.