Big Sur bluetooth keyboard not found after changing batteries. Mac mini

My keyboard battery’s got to 5% so I changed to new ones. Keyboard was not discovered by mac so I restarted the machine. I now have the login asking for a password and the bluetooth setup assistant telling me the keyboard is not connected and to make it discoverable.
I have tested the keyboard on another mac mini ru ning high Sierra and it works fine.

I can’t enter a password to get the big sur mac mini started – any suggestions on how to proceed ?