What it says in the title. And to further elaborate on the issue, I will recite the actions that were made prior to his disappearance:

1) I made it to the boss arena
2) Boss fight starts
3) I start getting my ass kicked and bail out by exiting to main menu in order to rethink my approach
4) Once I get back to the boss's arena, it's completely devoid of enemies, meaning I'm softlocked out of progressing in the game

So is there anything I can do to un-screw this aside from restarting the entire game?