You're resting in a cryo chamber on the Pillar of Autumn when the alien enemy army, the Covenant, starting invading and attacking the ship! Apparently, their goal was to finish you while you're in cryostasis. But your allies were quicker and managed to get to you awake befofe the Covenant even stepped foot on the ship. You step out of the chamber only to see Commander Keyes in front of you.

C. Keyes: Sorry to wake you early, old friend, but the Covenant is here and we need your help...