imac can't find my wireless keyboard

My bluetooth keyboard was working with my iMac, then the power was turned off accidently.

When the power was restored, the keyboard no longer works.

I followed the instructions in support, even to the point of removing the keyboard in system preferences.

When I restart, although the keyboard setup dialogue box opens, it can no longer find my keyboard.

Fresh batteries, in the right way, green light comes on for a second then goes off.

Bluetooth devices are marked discoverable. But no rew