I was playing my guitar when the Low E string went limp off the side.
At first I thought it was the tuner, but I actually saw that the E string took half my nut with it.
How would I go about fixing this?

must..not..laugh..at..."broken nut"..
i would probably just buy a new nut.. or i guess you could try gluing it back together if your pressed for cash
this has happened to me. you're going to want to, at least in my opinion, replace the whole nut.

get a professional to do it, take it to a local shop or guitar center.
that happened to me once...i super glued it back on. but if you're buying a new nut i suggest you take it to someone & let him put it on, an improperly set up nut can ruin your action. a friend of mine put his on himself, it was lop-sided & you had to set the 1st string really high else it was unplayable