What was happening when grendel approached the mead hall?

Beowulf and his men fall asleep in Heorot, and Grendel attacks them, devouring one of Beowulf's men. Beowulf, who was pretending to sleep, leaps up and grabs Grendel's hand. They battle until Beowulf tears Grendel's arm off, and Grendel flees to die.