What happened on your most unhappy birthday?

I stood up to my then partner's abuse. He told me I had no power and he would make sure I was left with nothing if I ever left him.
I knew if I cried he would use it against me and try to convince me I was crazy and emotional. He would do that so I would know my place. So I held my tears and bit my tongue.

30+ people invited, 1 showed, mother's friend's kid. (I was ~6-8), parents didn't do birthday parties for me after that. Next birthday get together I had was @16 with 3 friends. I'm not an only child but it was just me in that situation.

My 16th birthday we were on a family vacation. I was upset that day cause I wanted to be with friends, my dad was pissed cause I was unhappy and didn't even tell me happy birthday. Sounds like not that big of a deal but to teen me it was devastating

My 5th birthday. I had mumps, but my mother had already organised a birthday party for me, so I lay upstairs, confined to bed, listening to a roomful of other kids having fun downstairs.
This was a very long time ago, before children were routinely given the mumps vaccine.

These happened on different years but a really close friend died, one of my grandfathers died and my childhood dog died

Damn, reading the comments here really puts things in perspective. I was gonna say something about getting a bunch of Rick and Morty stuff despite having no interest in Rick and Morty, but damn y'all

The 9/11 suicide attacks. I didn't know until a couple days later that a relative died during it.

My dad fell quite ill after a blood transfusion (had been fighting CLL for ~25 years). He died 2 days later.

An actual hurricane took out my power for a week and blocked roads near me for days.
- Are you talking hurricane Milton or a different one? I have really not been thorough too many hurricanes, but Milton was absolutely wild to me. Couldn't even get gasoline for a solid week and some people I knew didn't even have drinking water. It felt mildly apocalyptic.
  
  A different one, although I'd prefer not to dox myself. Thankfully we still had running water (although those with well water did not), but I know what you mean about gasoline. The national guard had to be posted at gas stations as they came back online.

Not as bad as others thankfully but mine is: the world shut down due to a global pandemic.

Had a really bad flu, but was supposed to turn up to my first day of apprenticeship. Awkward enough to be introduced on your birthday, but I also looked and sounded like shit. Could also not relax in the evening, because my now ex had a broken collarbone and could not shower without help, so I had to drive to his flat. He had no one else to help him. Got no thanks for that.