Seized Engine After Transmission Replacement

So I had a mobile mechanic stop by for a weekend and replace the
transmission in my vehicle.  He said he was able to get the job done very
easily and didn't run into any issues.  After everything was put back together,
the car did not start (When it was before the job started) and you could hear
the starter clicking but not being able to turn the engine.  I went to check the
crankshaft pulley a couple days later and was unable to rotate it in any
direction.  My engine is surely seized.  Is there an idea on what exactly could
have possibly happened to the engine while the transmission replacement
was going on?  It'd be hard to bend any valves without being completely
inside the engine, right?  I'm hoping to see if I can't get him back here to
maybe do the job over again and see if he can't see where the mistake
happened.  Maybe there is something I can do for a possible simple fix?