what would cause engine to die while driving about 50mph then wouldn't start?

Could be any of a number of reasons....loss of fuel pressure,(fuel pump failure). loss if spark...ignition controler, loss of electrical power, alternator ....just to name a few.

Could be anything, pull the codes for the check engine light if on, then go from there. Check your fuel injection wiring harness to make sure it's not burned through from touching the exhaust manifold.