How can i fix it?

With the radio switched on , simply disconnect the battery, ignition will need to be off at this point.

Undo battery but first start engine while terminals are still connected. Only then lift off wires from battery.
Then simply with battery out of sight and engine running without battery hold the two wires that came off of battery together.

Then replace battery and refit in reverse order, this will have reset the radio circuit.

This always happens if battery goes flat or is disconnected.

Let me know how you get on

best regards

S Stewart

Roebuck nissan
June 2009