Since you did not mention the year of the vehicle, I cannot be sure of the power/ground connection. However, if you are sure of the connections, let us proceed. Check the constant 12 volt for power. Check the accessory power connection for 12 volts when the key is on. Check the ground connection for a good chassis ground. If you do not have power at these connections, check your fuses at the vehicle fuse box and at the stereo head unit itself (if one exists). If all power and ground connection are good and all fuses are good and, your unit still does not power on, check and make sure that the pins in the stereo harness are properly seated.
If all of these things check out as good then the issue is with the head unit.