iMac keyboard suddenly not working

Hi. I had to check a breaker for an unrelated issue- when i came in i needed to power up the iMac, which i did and it went perfectly as always. But after power up, My keyboard doesn't work. Note- I have plugged keyboard into MacBook and it works perfectly. I have plugged my external hard drive into iMac and it registers as it should. But for some reason, the keyboard wont function in any of the iMac usb ports. Help!?!