Complex sentence: The concert was shut down after the first song by the fire authorities due to overcrowding-it was estimated that 20,000 individuals were either in the arena or trying to enter it, when the capacity was roughly half that.
Split into simpler sentences: The concert was shut down after the first song by fire authorities due to overcrowding. It was estimated 20,000 people were either in the arena or trying to enter it, when the capacity was roughly half that.