2003 Chevrolet Silverado battery drain issue

I was driving and the dash started to flash 'battery not charging " and then a min later I lost all power lights hazards brakes and power steering. I checked the terminals and the grounds and I check a fuse. I'm pretty sure its my altnator but unsure. Truck now won't start and will not even lock the doors. Dead dead. Please help me understand how to get this fixed.