our 2003 chevy Silverado 2500 lt w/duramax diesel started great yesterday today won,t even turn over???

Sounds like a weak battery or bad starter. Check battery connections and grounds. If all looks good, I would test the starter.....to test for bad starter, jack up truck and find the starter, then have someone hold the key to the ON position like they are trying to start the truck. As they hold the key, rap on the starter with a hammer. If it suddenly turns over, the starter is bad and needs replacement.

I'm guessing you have a dual battery set up hooked up parallel since it is a diesel. I'd suggest you get a multimeter and check the batteries separately to see if both batteries are still good or whether one isn't pulling it's load