The title What Happened can be taken two ways: “This is how it went down,” and, “How did this unexpected, horrendous, and still mystifying result obtain? WTF Happened?” Clinton covers both, and is finally able to mention, now that the election is done, the role gender played.
In addition to the female factor, here is the short list of reasons Clinton enumerates that caused her defeat: race, the pseudo scandal of her emails, voter rage and desire for change, the media, fake news, the Russians, Comey, and Bernie. Plus, it is rare for a party which has held the presidency for two consecutive terms to win a third. Moreover, as Clinton points out several times, she won the popular vote—as did Al Gore in 2000—suggesting that the electoral college is a poor mechanism for expressing the national political will. To the above autopsy, add Republican voter suppression and gerrymandering.