91 Taurus not starting

Over the past few weeks, when I try to start the car, it took longer and longer to turn over. It seemed like it may have been a battery issue because it seemed to drag, but we checked and the alternator and the battery seemed ok.
Well yesterday, the car will not start at all. When I move the ignition, I hear a sort of buzz, and then nothing. The lights in the car stay on, and we even connected a one start battery and it did not change anything.
Does anyone have any suggestions? Also when I had been starting it lately I heard a squealing for about 10 secs that stopped if I moved the car forward a bit and then stopped. I had a mechanic look at it, he said he felt the serpentine belt might need replacing. Are these two problems related? Thank you for any help, I will have to tow this somewhere to get it checked out but I wanted to see what I can do on my end to maybe fix it beforehand.

I have gotten a suggestion or two it might be the serpentine belt, but I thought the belt just run the electronics and had nothing to do with the starter or engine.