The serpentine belt is not turning and nothing is turning that is attached to it?

Three possibilities, have someone look down at the main crank pulley while someone tries to start it, does it turn? If it isn't the pulley might be damaged or the crank snout could be broken. The next item would be to look at the belt to see if it is broken but normally if it broke while the engine was running the belt would have fell off. Last would be if one of the accessories like the power steering pump or idler has frozen and it is preventing the belt from turning.
To see if it broke is easy, grab the belt and see if it comes off in your hand. If the issue is a locked accessory you will need to use a belt tool or ratchet to release the tensioner and release the belt, then you can try to rotate each of the items it drives by hand.

https://www.2carpros.com/articles/replace-serpentine-belt

If the belt broke, then it could have been just age or a frozen pulley. So, check each of the pulleys to be sure there isn't a problem if it was broken.
If an item was frozen it will need to be replaced.
If the lower drive pulley isn't moving it could be that it has gotten loose or the crank has failed, the repair there will depend on what the issue is. Look things over and see what you find.