Lose an election.
I voted for Hillary Clinton, unenthusiastically yet happily. I would have voted, with similar feelings, for Bernie sanders, had he won the nomination. (I didn't participate in the primaries; Clinton had the nomination close to bagged up by the time Colorado had its caucus, which saved me from having to make a decision.) I would encourage all the people who are criticizing Clinton for criticizing Sanders to just stop. Her book tour will be over soon and this book won't be in the news anymore; that's why I had to publish part 12 of "50 Ways to Get a Book Deal" right now!
I haven't read What Happened yet, but I can enumerate the main factors that led, not to Donald Trump's victory, because he didn't win shit, but to Hillary Clinton's loss. They are, pretty much in this order:
6. Moral synecdoche*.
8. Fox News.
11. General right-wing and left-wing anti-Clinton hysteria.
12. General right-wing and left-wing anti-Hillary hysteria.
14. Party loyalty.
16. The judges.
17. The word "mansplaining." Anyone still using that dumbass word should be regarded as a covert Trumper or a Russian mole trying to destroy feminism from the inside.
18. Hillary Clinton/Hillary for President campaign team
19. There was a study ("Am I normal? A systematic review and construction of nomograms for flaccid and erect penis length and circumference in up to 15 521 men," italics mine) that examined penis sizes across the globe that was published too closely in time with the first American election in which a woman won a major political party's nomination for president. Honestly, this is probably a top-fiver, but I didn't feel like messing with the order again.
*This one probably needs a definition. Let's consult the fourth edition of The Penguin Dictionary of Literary Terms and Literary Theory, in which J.A. Cuddon defines synecdoche as "A figure of speech in which the part stands for the whole, and thus something else is understood within the thing mentioned. For example: in 'Give us this day our daily bread, 'bread' stands for the meals taken each day." In moral synecdoche, opposition to abortion stands for an individual's possession of and adherence to a complete and decent moral system, so even if someone couldn't reasonably be regarded as moral he can simply say "What about the babies?" and all of a sudden he's a fucking model of virtue, regardless of how many married women he's fucked or tried to fuck or how much abortion money he's thrown in the tear-stained faces of teenagers.
Author's note: This list is far from complete; I just got too depressed to finish.