Sometimes head units need a reboot or a return-to-factory-settings button to be pressed. Often, there will be a tiny hole somewhere on the face of the head unit. Into this, you insert a thin piece of wire or a toothpick (or similar) and press just hard enough for the screen to go blank and the unit to reboot. It sounds like you’re using the factory Mazda reversing camera with an aftermarket head unit, so you may need to visit a car audio specialist to get it all working together.

Changing a car’s battery can often be enough to scramble the settings on things like cameras and stereos. You may need to find the security code for the head unit to make it all work properly, too.

Assuming your car is fitted with a manual transmission, this could be as simple as a gearshift linkage that has jumped ship, or a selector cable that has stretched or snapped. If that’s the case, you should be able to look from underneath the car to see what’s hanging off or missing and not connecting the shifter to the actual gearbox. If the shifter moves freely inside the cabin, but doesn’t sound or feel like it’s connected to anything, that’s probably the case.

If you’re unlucky, you may find that there’s something inside the transmission (probably to do with the selector mechanism) that has broken, worn out or failed. In that case, you need to go inside the transmission to see what’s what, and that’s a much more complex task that involves removing the transmission from the car and lots of labour.

The automatic transmission is a mysterious, marvellous piece of equipment that manages to work almost in spite of its complexity. Guessing at a problem like this is fraught with danger, so the best advice is to get the vehicle to a transmission specialist, download the fault codes and see if they give the technician any hints as to what’s up.

Transmission problems with this make and model are hardly unknown, so avoid driving the vehicle any farther until it’s been looked at. You may end up doing more damage by continuing to drive it.

The actual problem could be anything from a transmission that is about to catastrophically fail, to one that has simply lost its 'memory' or programming and can be rebooted fairly easily and quickly.