If your wifi is not starting, you can have a shell access on the AR.Drone 2.0 by UART.
Depending on your motherboard version, the bottom connector pinout can vary (v1 / v2). You can recognize the version by looking at the number of pins in the bottom connector.
Note that you can damage your motherboard if you plug the UART in a wrong way!
If you don't manage to fix your issue, you can also change the motherboard, as the new one will come with a good firmware pre-installed.