My iMac stopped recognizing both my USB and my Bluetooth keyboards. Has anyone else had this problem?

About two-three weeks ago, I went into my home office where my iMac is located. (I normally work in the bedroom on my iPad.) No matter what I did, the USB keyboard that came with the computer wouldn't work. I tried changing USB slots with no success. I have three Bluetooth keyboards and the computer Bluetooth settings look fine,, but when I triied to type, nothing happened on the screen. I have no problem with my Bluetooth mouse.

Two hours later, the iMac "saw" the USB keyboard and worked fine...for about 30 minutes. After that, again, none of the keyboards worked. Any suggestions?