Q: I changed the batteries in my keyboard and turned it back on.  The green light came on for about 3 seconds and then started blinking.  But it won't type on the monitor.  Any suggestions?

Turn the keyboard off, remove the batteries, then spenc at least five minutes typing, pressing the flat of your hand on the keyboard, just randomely pressing lots of keys.

Then install new batteries and turn the keyboard back on and see if it pairs...the blinking light was a non-pairing issue.  Sometimes that can mean the capacitance switches of the keys need to be discharged.

If you keep the keyboard on a time and it still will not pair, can you borrow a USB keyboard from someone so you can login to the iMac?  That way you can go to System Preferences > Bluetooth and see if there is a reason it isn't pairing.  A likely candidate would be Advnaced and make sure all boxes are checked for Bluetooth.

Great, good to hear you are operational again.