Odd problem with compaq desktop

My computer has been having issues recognizing my keyboard until I have rebooted, well today, my cpu would shut off a few seconds after turning on. I unplugged everything except the power and turned it on and it stayed on like normal, I plugged in the monitor and everything was how it was, then I tried plugging in the keyboard and my cpu shuts off shortly after. Everything else works when plugged in.. I don't understand, any help will be much appreciated..