Car won't start after replacing left head and seting timing marks. Why? on 2005 Chrysler Pacifica

Car won't start after left head was replacedand timing marks were set at tdc. Why wont the engine fire? could it be the cam sensor?

The cam sprockets must be lined up with the marks on the timing cover and the crankshaft sprocket lined up with the mark behind the pulley on the cover. Also, check you Crankshaft Position sensor and cam Position sensors. they should have been installed with paper "shims" for proper clearance from the sensor tone wheel. The shims are available at your local dealership.