If your battery goes flat or you disconnect it for some reason, it triggers an anti-theft device on some car radios. When you recharge and/or reconnect your battery, the radio will not work until you enter a code or otherwise reset the unit.
Remove the detachable control panel if your radio has this feature. Some JVC models offer this. Insert the point of a pen into the reset hole. Press the reset button. This resets the unit but also deletes your presets. However, you can now reprogram the radio using the recommended method for that particular model. Many modern car radios have automatic station presetting via Strong-station Sequential Memory (SSM) or a similar technology.
Enter your security code if your radio is of the type that requires this. Some radios in Ford cars are of this kind, such as those in the Ford Focus. Every unit has a unique code. The code is linked to the vehicle identification number (VIN), which means the vehicle and audio system must match for the radio to function. Enter the code using whatever method is applicable for your unit. Then reprogram the radio likewise.
Enter 0000 if you do not know your security code or have never changed it. Some car radios ship with this security code. The Philips High-End RDS/EON Car Radio System CCR612 (V0.3) is one such system. The code will work if, like you, no previous owners of the car changed it. You can them reprogram the unit according to specific manufacturer’s instructions.