My Apple keyboard won't work unless its plugged in via the USB cable

Have you changed ports on your iMac?

Have you tried it on another Mac?

Try the following:

1 - boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

2 - Download and run Etrecheck. Be sure to give it Full Disk access before running.

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

If you mean Magic Keyboard try switching it on at the top right hand edge.

My guess is it's not connected to bluetooth.

Members are not going to just start making wild guess until you supply the requested details. My guess is little more then SOP.

Have you read ALL of this? Writing an effective Apple Support Communities question

Ok.

Guess # 1 - there are some users who want their moment in the Sun

Guess # 2 - there are other users who really what help on a Technical Apple Question and are willing to cooperate with requested additional Technical Information

We are willing to help but are going to need some Reliable, Actionable Information.

I can guess but do not think that is what you want. Is it ?

Writing an effective Apple Support Communities question