My 02 envoy won't start.

No it can't be shut down without the proper key, so your stuck with either the dealership(expensive) or a locksmith(still expensive but cheaper than the dealership)to get you vehicle keys cut right and the system reprogrammed.

This is a very common problem on GM vehicles. The security system is notoriously known to fail. Basically when you insert your key, the system checks for a certain signal. If the system cannot get the correct signal from the key, it assumes the key is incorrect or you are attempting to hotwire it. The system then goes into SECURITY lockout mode. (prevents vehicle theft) This will either turn off the starter, or the fuel injectors. This has been happening with GM's for a very long time. There is a company called Newrockies inc. that sells a bypass for this online. Works great.