Apple wireless keyboard won’t work; locked out of iMac

Guys

For some reason my Apple wireless keyboard has unpaired itself from my iMac, first time this has happened, and now I’m locked out of my iMac... Any ideas...?

I’ve changed batteries and checked the battery connection etc and that all seems fine... Do I need to borrow a USB keyboard to then be able to log back in and pair my Apple wireless keyboard?

Help please!!