Wireless keyboard not connecting via bluetooth - was working 3 hours ago

I've tried most of the fixes being recommended but nothing is working.

My wireless keyboard was working this morning, now it's not. Magic mouse is fine. I've just paired the keyboard with my wife's IMac - no worries.

I've no idea what's going on here. The batteries are new, it's an old keyboard I dug out of my wife's stash yesterday when I got a monitor to work on - but that shouldn't play a role.

As I said, it was working well before and it's working on my wife's IMac. Now, it says "Could not connect to device".

I'm using Catalina on a 2018 Macbook Pro.

Any ideas?

Thanks in advance.