If you've disconnected the battery the radio will loose its security code.

Reconnect the battery and when you turn it on it should display --CODE-- on the window.
Using the channel selection buttons at the bottoms (if its a 5000 or 6000 model) keep pressing each button in turn until the code is entered.

for example if the code is 1234 press each button in turn until 1234 is displayed in the window, then when you press the fifth button the code should lock and you should be up and running (assuming you've put the right code in)

I hope that helps, and good luck.

If it's not the code, sorry but I'd be as stuck as you.