Is there a word for the moment when the ground falls out from beneath your feet?

Looking for a word to describe that moment when you realize everything is falling apart. You are going along in your life and everything is ok and then suddenly, you realize it is really not. This would be for an event akin to you think you're happily married and then your spouse files for divorce; you think your boss is a great, upright person and then you realize they were embezzling money and the company is folding, etc. Not so much for a natural disaster, if that changes any potential answers,

In some cases, cataclysm (“A sudden, violent event”) will work. Also consider:
• crisis (“A crucial or decisive point or situation; a turning point”)
• turning point, “A decisive point at which a significant change or historical event occurs, or at which a decision must be made”
• crossroads, “A decision point; a turning point or opportunity to change direction, course, or goal”
• watershed, “A critical point marking a change in course or development”

Perhaps a bit wordy, but peripeteia?

per·i·pe·te·ia also per·i·pe·ti·a (pr-p-t, -t)

n. A sudden change of events or reversal of circumstances, especially in a literary work.

[from Greek, from peri- + piptein to fall (to change suddenly, literally: to fall around)]

We might say that person was blindsided

catch (someone) unprepared; attack from an unexpected position.

Catastrophe

A sudden disaster or misfortune (Chambers)

How about wakeup call:

an event that warns someone that they need to deal with an urgent or dangerous problem (often + to do sth) The 1971 earthquake was a wake-up call to strengthen the city's bridges. (often + to ) The World Trade Center bombing has served as a wake-up call to the FBI on terrorism.

Colloquially, you could also say an oh shit! moment.