Can't pair bluetooth keyboard with iMac

Suddenly today my bluetooth Apple keyboard (2003) could not be paired with my iMac (OS 10.11.6). It connects, but cannot be paired. For a while, I was getting a box with 4 zeros in it to input a code, but I had no idea what the code was. Now I get nothing but the 'failed' message. Just put in brand new batteries. Used to sometimes lose the pairing and a code (five or six digits, I think) would show up on my screen, I'd type it in on keyboard and be all set. Nothing has changed between yesterday and today... except that I can't pair the keyboard. Something tells me it's the new normal of built-in obsolescence. Thankfully, the old USB keyboard that I still have from my lime green original iMac still works just fine. 🙂 Any suggestions would be appreciated!