1) Check your battery. Make sure it is properly connected and has enough power.
2) Check your fuses. Look for any blown fuses, especially those related to the stereo or ignition.
3) Check your wiring. Make sure everything is connected correctly, especially the wires that you've had to work with while installing the new stereo.
4) Try resetting your vehicle. Sometimes, disconnecting the battery for a few minutes and then reconnecting it can help resolve electrical issues.
5) Check the ground wires going to your truck battery and from your truck to the engine block. Be sure they are securely tightened and not corroded.
6) Check the ignition switch. Make sure the switch is making proper contact and is not damaged.
