The term for alcohol-related memory loss is "blackout," not the same as passing out. Complete memory loss is rarer than partial loss. From the article Alcohol Induced Blackouts: The second type of blackouts, fragmentary blackouts, as the name suggests, involve partial blockade of memory formation for events that occurred while a person was intoxicated. Goodwin and colleagues(1969a) reported that subjects experiencing fragmentary blackouts often become aware that they are missing pieces of events only after being reminded that the events occurred. Interestingly, these reminders trigger at least some recall of the initially missing information. Research suggests that fragmentary blackouts are far more common than those of the en bloc variety (White et al., in press; Hartzler and Fromme, 2003; Goodwin et al., 1969b).
This page summarizes some of the impacts on the brain in more detail. Alcohol-induced blackouts: Alcohol and the Hippocampus.