Does it say "safe" when you power it up? If so, you need to enter the unlock code. It should be in your owner's manual packet. If not, you have to go to the dealership with photo ID, registration, and title. They can look it up based on the VIN number of the car. They may charge you up to an hour's labor, so be sure they write it down for you in the event you have to disconnect your battery again in the future.