i have a 2008 dodge ram 2500 with a 6.7 litre diesel. last evening i changed starter without unhooking batteries first. when i went to start truck it wouldn't crank over. lights come on dash lights, power windows etc. work as they should but truck won't start. i changed starter relay and checked fuse box under hood but nothing. still won't crank over. i have power at starter but won't respond when i turn key.

 

Im guessing you probably got some sparks when working on the wiring for the starter ???
Anyways...for whatever reason sounds like the starter isnt recieving the 12v signal essentially from the ignition switch in the start position. it is possible there is more than one fuse for the starter. I would be closely inspecting fuses again and even further testing each one..paying closer attention to start and run fuses.