Why would I need to reset my computer for it to start

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 are ways to bypass this now. Google NEWROCKIES INC.

My check engine light came on because my gas cap wasn’t tight