My 2001 catera out of the blue stopped running. I changed the crank shaft position sensor . I have been told it could be the gas cap preventing it from starting. the code P0455 comes up

SOURCE: crank sensor

Yes your crank sensor tells your computer when to fire the injectors and the spark plugs so, if you have no crank sensor, you have no start. It could prevent your car from starting depending on the conditions when the security system was activated. Has anyone kicked the rear of your car lately? Or have you been in a fender bender? Those 2 things could trip the inertia switch in the fuel pump which could also cause your car to not start like that.

SOURCE: 98 Cadillac Catera, Cranks but not starts.

could be crank or cam sensor

SOURCE: Need to replace both crankshaft position sensors 2001 Cadillac

4.6 the twocrankshaft position sensors are located on the front of the block just above and to the left of the oil filter. not hard to replace.

SOURCE: gas tank won't open, trunk won't open, radio stopped working, left tail lamp won't work

It's the body control module. It's located under the dash to the left of the steering column. I had same problem, went to the junk yard and bought one and the problem went away. Dealer wants $600.00-700.00 for the BCM, I paid $30.00

SOURCE: how do i fix my 2001 cadillac catera idle problem

The problems you are describing indicate to me that you likely need to replace the coil pack that runs from the spark plugs ,and the plug wires end up at it.It also may be the spark plug wires,get a water bottle and spray the wires after dark and if you get a light show from them then replace them,if not,replace the coil pack.