why has engine failsafe program light come on and now will not let car start?

Hello, A classic problem with these vehicles, often denoted by the 55C4 code, is the ABS/DSC module. The good part is there are rebuild services out there that cost far less than purchasing new, but the part must be removed from the vehicle and later reinstalled. Aside from that, it seems you have a vacuum leak, which is most common of the intake manifold gasket on the 3.0l, but I'm not sure which engine you have. You can find the vacuum leak by spraying a mist of water around the running engine until the engine changes its sound. This will let you know the are the vacuum leak is in.