To feel down or disheartened; to get depressed (colloquial). Can also mean to be dented or become concave in a literal sense.
試験に落ちて少しヘコんだが、友達と話して元気を取り戻した。
I was a bit down after failing the exam, but talking with friends cheered me up.
上司に注意されてヘコんだが、翌日には気持ちを切り替えた。
I was disheartened when my boss reprimanded me, but I changed my mindset the next day.
彼は人からの一言で簡単にヘコむタイプだ。
He's the type who gets easily down because of a single remark from others.
駐車場でぶつかって車のドアがヘコんだ。
The car door got dented after a bump in the parking lot.