I fell asleep watching our daughter?

My wife went out for the night so she stuck me with our one-year-old daughter. she and i played for a while, had tickle fights, had tea parties, watched some movies, danced, karaoked, etc. it was a nice time after 3 hours though I was exhausted so I fell asleep, i had just changed her diaper and put her down for her nap on the floor. well i was awaken by my wife five hours later and she was furious. she said she came in and our daughter was crawling around and i was asleep! i guess i messed up but my wife is threatening to leave me over this. my daughter was fine and was smiling and giggling so.... it'll never happen again, why is my wife being so mean and unreasonable about this? Stuff happens. get over it.