Power issues or alternator?

On Thursday I left work, and the car started making the undeniable power steering whine. I turned off the A/C and rolled the windows down and she was fine. After about a mile or maybe two the instrument panel and radio went dark. I mean gauges didn't work, no lights, no radio, nothing. But I could still accelerate the car. When I came to a light and slowed down, the car stalled and there was no power at all. I pushed it to safety and tried again. I had power but when I went to turn it over, I got the dead battery grind. What could be my issue? I should add, the belt seems a little loose as well.