The game itself froze. After some time I saw the mouse cursor show up with a busy icon. I think the game was trying to either trying to load something or taking too long to process everything. Either way, when I tried quitting out with alt-f4, a windows message popped up saying it'd stopped responding, so whatever the game was doing was preventing it from responding to windows events (as is typical if loading stuff or getting stuck in a time-consuming algorithm).
I tried it again. The music still started playing before the game did, but loading didn't take anywhere near as long this time. I also managed to play fight the enemy this time. It's possible my computer was trying to do something in the background without telling me (which windows will do sometimes to my annoyance).
I think if you continue to work on this or make anything similar to this, the first two things I would suggest adding is an indicator that the boss is taking damage (either by adding a boss health bar or by adding some visual of the boss in pain) and a change in gun position when the player is shooting so that the gun is aligned with the player's shots (thus making the shots look more like they came from the gun).
