Car will not start after replacing serpentine belt

Okay, then it is not locked up.
It should start even without that belt on. That is a belt to turn accessories like the alternator, etc and it's not a timing belt that turns the camshaft.
So, now back to why it will not start.
If there are no lights on, the next thing to do is either check for a spark, or if you have some starting fluid, see if the engine will start and run for a few seconds after spraying a few small shows of starting fluid into the throttle body and then quickly making it to the ignition key and try to start the engine.
If it does start and run for a few seconds, then at least you know the ignition appears to be okay and if the fuel pump does run, the fuel injectors probably are not working and could be a blown fuel injector fuse, a blown PCM fuse, failed crankshaft position sensor or a security issue, but the check engine and/or security light should rule that out or not.
If it still does not start with the starting fluid, then start off with checking the ignition fuse/s.
Now thinking about that gas in the oil I was assuming maybe the fuel pressure regulator diaphragm may be busted allowing the vacuum hose to suck gasoline into the crankcase ending up in the oil pan. Remove the vacuum hose from the fuel pressure regulator located on the fuel injection manifold assembly and check if there is gasoline in the end of the vacuum hose, and if there is then that is why there is both gas in the oil and the fact that the engine will not start because there is no fuel pressure.

I hope you did not miss the editing.
Now thinking about that gas in the oil I was assuming maybe the fuel pressure regulator diaphragm may be busted allowing the vacuum hose to suck gasoline into the crankcase ending up in the oil pan. Remove the vacuum hose from the fuel pressure regulator located on the fuel injection manifold assembly and check if there is gasoline in the end of the vacuum hose, and if there is then that is why there is both gas in the oil and the fact that the engine will not start because there is no fuel pressure.