Replaced battery today due to the previous being dodgy. Initially started up without an issue. I then left the car for the afternoon, when I got back to the car it failed to start... wouldn't crank, ignition lights turn on but nothing happens. A new message had also appeared "transmission service required". I have tried locking, unlocking key fob several times, I have disconnect battery, tried the old battery back in there. All with no luck. Any ideas?

Incase anyone has this problem, the computer had ****ed up and started bringing up loads of fault codes due to changing the battery. Called garage out to my car £50 later faults cleared and it worked again.