I've had hallucinations and out-of-body experiences since been very young, and severe crashes into hopeless depression and emotional turmoil from the age of around 16. But I'm guessing that you mean a episode that instilled serious fear, confusion and panic - an episode of nightmarish existential despair. For me that was when I was 21. I was homeless at the time and into drugs. I hung around with "friends" who were also drug users. After a night of taking speed and going without sleep, I then the next day smoked some weed. The combo of speed-comedown, cannabis, poor living, and been surrounded by people I could not trust was the trigger. Reality shattered. I can only remember fragments of what happened that night.

Thinking that people were trying to poison me, plotting against me, making fun of me.

Going into the kitchen to drink from the tap rather than what was offered.

Faking unconciousness like a mongoose so that people would leave me alone.

My "friends" trying to bundle me into the car and my subsequent escape attempt.

Finding the door locked and having to fight my way through the crowd around me.

Hiding in the bathroom, under the sink, tears, snot and blood running down my face from where I clawed my face.

People gathered around me with ear-pieces and invisible pumps that controlled the levels of the chemicals in my body.

Hyperventilating and almost passing out when people got me out of the bathroom.

Been taken to someone else's house where I fell unconscious and finally slept, dreamlessly.

After that I had periodic attacks of less intensity for several months, and it took me around three years to truly overcome the shock of that night.