Replaced the thermostat on my 95 RAM 2500 V10. Reached behind alternator with socket wrench to tighten bolt while engine running. Big spark. Engine died. Now won't start. Cranks fine. What did I knock out?

Check your main fuse links. My guess (without looking at the fuse block) is probably the 50 amp main that blew. It controls the power to a lot including the ignition system and charging system. It's a place to start, but I'm not positive that's the problem.