I know some people use a tool that holds the timing chain in place when they have the head off. You didn't do that with one of those and forget about it, did you?
Otherwise I don't know what it could be. Id hate to think you dropped a nut or something down there on accident and now its jammed in the timing chain/crankshaft.
Make sure your starter is all good, and if it is, then you are going to have to go through the heads again because idk of anything that could cause that but be fixed without taking it apart. I hope im wrong, but who knows