Blacking out from alcohol?

What happens physically when u black out? Ok, I may have a bit of a drinkin prob.. I've only been drinkin for about a year and a half, but its always been beer.. About 8 months ago I started drinkin everclear, then I moved to bacardi 151, mixin it wit coke zero (diet thing haha).. So this past Saturday night I apparently blacked out for about 3 and a half hours.. We were havin our usually party at mine and my fiance's house so I was in no danger, everyone there is in our "group".. I've had instances where I dont remember goin to bed or remember gettin there jus not fallin asleep, but never have I not remembered something like this.. Everyone said I was walkin around, playin games, smokin, and that I jus seemed really fuked up.. I'm a 20 yr old female, I'm 5'9, weigh about 170, and I drank lil over 3 quarters of a fifth of bacardi that night, which is more than I've ever drank.. I dont get it, wat happened, how come I cant remember? What physically happens when u r walking, talking, doing things and can not remember them even fuzzily? I hope someone can answer my question....