The dilemma is, of course, whether the damage was done while at the shop, or the damage was done sometime afterwards. If you had noticed the damage while leaving the shop, there would be little question as to origin. But the more time that passes between leaving the shop and discovering and/or bringing the issue to bear, the more tenuous the connection to the shop, and the more likely the damage was done later.
I am reluctant to recommend legal recourse because, in my opinion, a good lawyer won't do anything you can't do yourself. Except charge you for it! [I used to practice law.]
I agree with the previous gentleman's recommendation: You are not in a position to demand much of anything. However, an open, honest communication with the owner about the problem will, at a minimum, make him aware of a potential serious business problem and may, in the spirit of goodwill, get the problem fixed.