If the battery has gone flat or been replaced , then the radio might fail to work because it will usually need a Security Code to be entered. This code should be noted somewhere with the car`s paperwork.
To enter the code see the advice + demo here
http://www.davidsonelectronics.co.uk/code_entry.htm
If you can`t find the code then ask a NEW question quoting the radio MAKE MODEL and SERIAL NUMBER.
This info is on the radio casing.
Instructions how to remove from dashboard found here
www.tinyurl.com/4ydhop
RegWorts
August 2008