USB ports on my iMac are not working

https://support.apple.com/en-us/HT204063

If for some reason you are still having trouble with it, please Google Reset NVRAM or PRAM on your Mac

First thing to try is new cables, they can and do fail sometimes. Also restart the computer and while at it reset the NVRAM per the directions in https://support.apple.com/en-us/HT204063.

Post back if this does not help.