OS X Yosemite - My wired keyboard does not work anymore
I experienced the exact same issue when I downloaded Yosemite.  I spent well over an hour with Applecare troubleshooting, PRAM resets, everything.  Tried all the different ports -- it was a mystery and they said I had to bring it to the store.  So I shut the computer down and brought it to the store a few days later, and miraculously the keyboard worked on the first start.  I felt like an ass.  Just this morning I downloaded the latest Yosemite update, and the exact same thing happened.  I tried all the proper troubleshooting as last time with the same non-results.  This time I said, let me shut my computer down for a few hours, unplug the keyboard, and basically simulate what I did last time at the store, but without the hassle.  Sure enough, I just restarted and re-plugged the keyboard in and it works again.