Accidentally smoked spice and missed my exam, what can I do?

I'm in third year and have had a disastrous situation. Went out on the Wednesday night, my exam was on Friday afternoon. I had done a lot of revision and so I thought I deserved to go out on Wednesday. Long story short I lost my friends and ended up dancing with a group of strangers, who then invited me back to theirs for afters. When I got there a few of them were passing around what I assumed was a joint. They asked if I wanted some and as a weed smoker I of course accepted. I don't know what was in that zoot but it sure as hell wasn't weed because 2 minutes later I was standing up on the sofa, I felt like my whole head was on fire and my brain was screaming. I was vibrating and I couldn't sit down. As time wore on the feeling became even more intense, I felt like I had been transported to hell, I saw the faces of demons around me.

Eventually I must have left or they kicked me out because the next thing I knew I was running down the street at 5am. I was paranoid that the police or someone else would find me in this state so I decided I needed to find somewhere secluded to ride it out. I went into a park and found a patch of trees and just stood there. I felt I had too much energy to sit down. I was drifting in and out of reality, I would periodically come to and feel somewhat sober, and then fall right back into a trance for hours. I remember it getting light and dark again. The next thing I knew I woke up lying on the ground with a pounding headache and absolutely starving, the most ravenous hunger I had ever experienced. I looked at my watch and was dumbfounded. It said it was 6am on SATURDAY MORNING. I had gone out on WEDNESDAY NIGHT. I thought my watch must be broken or something so I dragged myself back to my house barely able to walk and my housemates were shocked and furious with me. Apparently I really had been gone for 3 DAYS and most importantly totally missed my exam on Friday. They had been about to call the police. My parents thank god didn't know anything because I'd spoken to them on Wednesday so 3 days without speaking to them wasn't unusual.

My question is how the hell do I tell the uni what happened. Do I even bother? I'm sure smoking spice and standing in a forest for 3 days doesn't count as extenuating circumstances.

Edit: After reading many of the comments here (the helpful ones and not the ones telling me to work at mcdonalds or admonishing me for being a user of the devil's lettuce, I'm sure you guys are a great laugh) it seems the most likely explanation is that whatever it was I smoked triggered some sort of psychosis as a result of my already poor mental health.

Edit 2: lots of people saying my entire post history is that of someone who is nuts. I never gave it much thought but now i see it from a third person perspective yeah I've clearly got serious problems which are exemplified here. However, I would like to add that the piss drinking story is quite obviously made up. But the others are all real, I really am a drug addicted savage with no real prospects.