Many cars have a security feature that disables the entertainment system when battery voltage is removed. You may have to get code from dealer to get it working again. Sometimes I have found security code written down in cars owners manual. I have listed the procedure below and it mentions a code card that could be with owners manual.
If the radio is disconnected from the power supply, it is not operational when re-connected. After switching on the ignition and the radio, the readout CODE appears in the display. The code number for the radio is advised to you on the enclosed CODE CARD. To enter the first digit of the code number, you have to continue pressing station button 1 until the correct digit appears in the display. Adopt the same procedure for the second, third and fourth digits. Once you have entered the correct code number with the station buttons 1, 2, 3, 4 and confirmed the entry with the SEEK button (press down), the readout CODE disappears. The radio can now be operated. If you enter a wrong code three times, you must wait 15 minutes before making a new entry. In addition, if 9 incorrect entries are made, the radio will then not operate for 24 hours. During this time the radio and the ignition (terminal 15) must remain switched on.