Truck won't start

I have a 2001 chevrolet 2500 hd duramax diesel with allison auto trany. Just bought it
but has been running great for 6 days now. I came out of the store and went to start it
and there was nothing...no lights, every thing was dead. I had power to the starter and
when i crossed the posts at the starter it would spin the motor but not start. After
messing around for half an hour the security light comes on and the key bell rings but
when I go to start it all goes dead again for another 15 to 20 mins......hhheeellllppppp