My 2005 Honda Rebel 250 won't start, can anyone

I drove a couple hours to look at the bike, brought a trailer to tow it since I just got my license and wasn't comfortable driving it all that way yet. It was driving fine, no problems, no leaks. We tied it down on the trailer and I towed it for about a 2 hour drive. When I got it home there was a small puddle of fuel under the engine coming from one of the black hoses that sticks out underneath. Now it won't start. Before someone asks, YES I had gas in the tank, the clutch in, kickstand up, killswitch to "on", fuel switch on, key in, turned to "on" and the engine in neutral. What would cause this and how can I fix it to get it running again? I was also told it might be an adjustment to the altitude since I went up more than 4,300 ft. Could this be true?

Best thing for you to do is buy a maintenance manual for the bike - all of your Q's now + future will be answered in the manual

Kill switch on the handlebar in "run" position?

Something's flooded. Try to start it again tomorrow.
If you ran down the battery trying to start it already, then you should put the battery on a
battery tender junior (or equivalent).

The puddle is fuel that was spilled out of the carburetor overflow hose.
The spilled fuel probably entered the cylinders and flooded the engine.
Remove the spark plugs (there should be a spark plug wrench in the bike's tool kit)
Press the start button for a few seconds to try and start the engine.
The cylinder compression will blow any excess fuel out of the spark plug hole.
Dry off the spark plugs and put them back in.