2003 Cadillac CTS cant get the car to start

Everything is tight I just got a new battery yesterday so that ends good I just dont understand y the lights come one like head lights and in side dash lights but nothin else in or out of car works the key willnt even come out I think it might be something to do with the icm?Like is there anyway to reset the cars computer?

I am having the same issues as the person above, I have a 2003 cts and at first my engine light came on, took it to dealership to have code reset the car was fine for about a week. The engine light came on again and a couple of days later the car did not start at all, had it towed to dealership again and was then told it was the battery so the battery was replaced and the engine light code reset one more time, this time the car was ok for about a month, this past weekend the same trouble, the car would not start, completely dead, no lights, no radio, nothing. Did not take to dealership, took it to a mechanic looking at it, so far says something is draining the battery but he has no idea what it is, has had the car for a week and still no definite answer, can anyone out there help, or give any suggestions? Could all this be the codes? Any advice would be appreciated. Thanks