Ford 04 plate not working
 

Hi Guys,

I bought an 04 plate ford focus yesterday,
The battery had been dead a while so naturally it blocked the radio (not a problem i know how to unlock that) but when i pulled the radio out to get the serial number on the side i put it back in and now the radio doesn't even turn on?
3 plastic connectors attached to wires in the back are all pushed and clicked in properly, however there is another single hole on the other side at the back that doesnt seem to have anything in it, anyone have an ideas for me? :)