It seems like you had a orthostatic hypotension due to dehydration.
This occurs when you are standing upright with dehydration .Dehydration will cause decreased blood volume and increased blood in the lower part of the body. Standing also causes pooling of blood in the lower part of the body which causes decreased blood supply to the brain .This causes sequential shut down of the functions of brain which starts with hearing, sensations in the hands, nausea, dizziness and then unconsciousness.
I would recommend that you keep yourself recommended in situation that may cause loss of fluids like outdoor sports, tanning sessions etc. If you feel that you might have an episode of hypotension, first ask for help and then sit for a while and drink water. It's very important to protect yourself from future episodes as the injury from fall can be pretty bad.
I would also recommend you to follow a strict regimen of diet and exercise so that you can condition your body to prevent future attacks of hypotensive spells.