Hey man, I've seen you post about this before and I never brought up my own experiences but I have you to thank for giving this phenomenon a name for me (Cotard's Delusion). I have schizo-affective disorder (schizophrenia + bipolar) and I absolutely experienced this 17 years ago when I went to the state hospital for like 3 months.
Immediately before that, I was catatonic in my parents' house where I was aware of what was going on around me but couldn't react, speak, eat, move, or do anything really but lay there in bed. Like I said, I was aware but couldn't do anything about it, like being trapped in a glass box. Anyways, my parents finally got sick of it and drove me to the ER. I guess they put me on an IV and knocked me out because my memory was wiped until I woke up on a gurney with my legs and arms strapped down being driven to the state hospital. When I was there I didn't talk to anyone for like a week, even when other patients approached me and tried to start conversations. I thought to myself, "So am I dead and in Heaven or am I a patient here?" Keep in mind I was off all meds and they wouldn't give me anti-psychotics for like a month until they put me on Risperidone injections.
So yeah, I thought I was dead for at least a month back 15 years ago. I never told anyone or even knew there was a name for it until you posted about it a couple months ago. Since then I did tell my psychiatrist about it but he said it was all in the past and nothing to worry about anymore. He's right but at the time I was scared as shit I would never see my friends or family ever again. That I would be trapped in the hospital forever and that place was more like Hell than Heaven.