Why did i black out when exercising?

There was a lack if oxygen to your brain and it was going to reboot itself. You were dehydrated lots of water will prevent that from happening. I'm curious to why getting on the floor helps though it happened to me a couple times at work and at a theme park I totally blacked out. At work it sounds gross but I had to lay on the cold floor away from the toilet by the door LOL to not black out. I was definently dehydrated each time it happened