Q: Why won't my car crank up??

I really hope that it is not anything serious. I pulled up in my driveway put the car in park and it cut off. I tried to crank it up but it would not start it was just clicking. The radio and my lights were still working. I had the battery and alternator tested a couple weeks prior and everything was good. Now my lights are starting to go dim and I think the battery is dying. I can't ignore the fact that the reason I had it tested was because of this loud noise in the front of the car. It was sounding like a F150 . It was my belt making a lot of noise what could be possibly wrong?