USB problem and power cord

I was messing with my home server machine (running Windows 8 Pro) by turning on and off an external USB hard disk dock for quite a few times in order to check and resurrect a broken hard disk.  After a while, I realized that the USB hard disk dock was not being recognized.  Eventually I decided to reboot the machine, only to find out that ALL USB devices were not recognized.  I kept turning the machine on and off a few times and the problem still persist.  None of the USB devices were recognized, including keyboard and mouse.

Eventually I found out a simple way to resolve the problem: unplug the power cord for a few seconds. That solved the problem.  Now all USB devices were being recognized again.

Another problem solved by trial and error.