I have a 2001 vw passat i changed th doors and it will not start what could it be?

I had this happen with a different vehicle. "My" issue was the security system. When I disconnected the wires going into the doors, it tripped the security system into the ignition lockout mode. I had to reset the computer (free and it only takes a few minutes). Mine was able to be reset with one of the code readers from O'Reilly Auto Parts which they will let you use for free. Some vehicles have a reset button built in. If you have a security system on your VW, first make sure all wires were reconnected and not broken during the installation of the doors. If everything checks out ok, then give your local VW dealer a call and ask to speak to someone in the service department. As the service person if there is a security system reset button anywhere on your car.