Removed factory stereo now security lock is on n I can’t stsrt my vehicle

Hi, this is Bo, an auto expert on RepairPal's Car Genius team! It sounds like your 2006 Buick Rendezvous is stuck in a security lock mode, which can happen when the factory stereo is removed. Since you've put the original stereo back, the ignition should ideally reset. However, if you’re still not getting any power, check the battery connections and fuses again. If the “3x key on-off” procedure didn't work, you may need to start the process over by waiting a few minutes before trying again. If problems persist, it might be best to consult a professional. If you need any further advice or want help finding a trusted RepairPal shop in your area, please feel free to call us at 877-913-1554!