GM radios have an extremely high failure rate and to insure they make as much money from you after the sale, they no longer allow us to buy radio service manuals or parts. In an effort to prevent you from buying a higher-quality aftermarket radio, they now build the Body Computer into the radio so you have to get it fixed or you won't have power windows or cruise control. They want every possible dollar they can get from you for their grossly over-priced repair centers.
If your battery was recently disconnected or run dead, that can also make the radio lock up. If you don't know the code to unlock it, the dealer can get a code out of it that can be translated by calling their secret 800 number. That will get them the unlock code, and of course, they don't do that for free. Either way, along with all of their other many customer-unfriendly business practices, their radios are designed to make them lots of money after the sale.