When turning on the vehicle, if your Denzo headunit give you the GMC splash screen, then gives you a blue screen with just the clock on it, no allowing you to navigate further, try the below steps in order. Moving to the next step, if the last didn't work:
Hold down the volume knob for 5 seconds. (As the volume knob is also a radio power button.)
Hold down the menu button for 8 seconds.
Try a CANBUS reset:
CAN-bus Reset:
Remove positive and negative cables from the vehicle battery.
Leave the battery disconnected for about 15 minutes
(if you have a diesel, disconnect both batteries)
Reconnect terminals.
With all doors closed, lock the vehicle and let it sit for 15 minutes.
Start vehicle with the door open.
Let vehicle run for 5 minutes.
If none of this works, please call Infotainment to have the radio physically looked into.