Seriously, like the other comment says, if you were unconscious for hours, you are more than likely to have a concussion.
If you havent already, go see a doctor asap.
And if you don't want to tell your supervisor, just be vague and say something like "I had an accident" or "I had to go to the doctor". I don't know what its like where you live, but in Germany that would be approximately the expected level of detail (combined with a doctor's note if you're sick/absent for longer).


The Styx as in ... the mythical river that runs through the underworld? Yeah, I can see that being a low crime area...
(Sorry, european here, if you meant someplace in the US, I've never heard of it)