If the 3v CR2032 display battery is new and there is still no display, it may be the battery cradle has become distorted when attempting to slide out the old battery, please follow the below to solve this:

As you can see, the contact is no longer flat and able to fully contact the surface of the battery as it has distorted upwards.

Press down with your thumb to 'mould' the battery contact back into a flat shape - This may take several attempts!

Insert the CR2032 battery and once the display has re-appeared now replace the 12v battery.