It's extremely unlikely anything catastrophic has happend internally whilst the engine was switched off, much more likely to be jammed starter motor pinion, seen it many times before.
Ask the garage to remove the starter motor and then try turning the engine by hand, if it's still stuck ask them to drop the auxiliary drive belts off and try again, I am almost certain it won't be the engine itself (stranger things have happened, but very rarely!)
Good luck and let us know how it goes.
Paul.
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