My Ford Mustang has been sitting in the driveway for about three weeks, which was the last time I started it and it sounded fine. I started it today and there was a loud squealing, my wife looked when I started it again and she said she saw the serpentine belt not moving. How do I get the belt moving?

 

I would suggest that you loosen the belt with the belt tensioner and attempt to rotate all the pulleys it turns. Except for the crankshaft pulley one of them is most likely frozen.
Once you determine which one you can go from there to effect repairs or replacement.
Just make sure the defrosters and/or A/C is off if you try to run it again before checking.

 

I loosened the tensioner and turned all pulleys, the only that did turn was the one labeled air pump on the diagram. Does this I have to replace the whole air pump?

 

Unfortunately, YES !

 

Depending on the year of your Mustang and your state's emissions laws, you could replace the air pump with an idler pulley. More power and cheaper.