Is the SECURITY light lit on the dash ? If so there is a problem with the anti- theft system . An a code is stored in what ever module controls it . There is a way to by-pass it . go to you tube an search GM VATS , Pass key , pass lock how to bypass .Or you can take it to a Chevy dealer or some other repair shop. Just depends on how much money you want to spend. And if you bypass the system your vehicle won't be protected from theft anymore !