What Causes Numbness In The Legs Upon Sitting On The Ground?

Hello, sometimes when I sit on the ground one of my legs falls asleep and i’m able to get up and stumble on it, but today when i was at my second day of my dance competition, during the awards session all the dancers sit in the stage and they give them ribbons, I was sitting and started to feel both my legs fall asleep by the end of the 20 minute award session when i tried to stand up with the help of a friend, i couldn’t feel my legs at all i looked down at my foot and wasn’t and it was bent and i couldn’t move it my friend let go and i stumble back and fell. I remember seeing my feet take three steps but I couldn’t feel them happening at all, after I fell i was stuck on the ground for two minutes, I tried to tell my legs to stand up but they just weren’t moving. I eventually stood up but was walking shakily then started to get piercing pins and needles then I felt them falling asleep again and I fell into a chair, while siting in the chair i tried to lift one of my legs just off the ground, I wasn’t able to. Eventually after a few more minutes in that chair the feeling came back but I would really like to know what is happening to me. I have 2 more days of competition and lots of Awards sessions left I’m terrified.