if you have a usb keyboard and mouse, remove any other USB devices. A short or drawing too much current will shut down the port and prevent it from working. (apple devices draw too much current and will be considered a short circuit on a PC unless you have a special USB port and driver)
Also, if you have a usb 3.0 port if the driver fails to load the device will not work.
ALSO, some systems have the ability to tell your USB ports to go to sleep, if that is the case you can not wake the machine with a USB mouse and keyboard. (disable the sleep setting for the USB host controller in BIOS or in the windows control panel power management)
- you might also move our keyboard/mouse to a different USB controller (USB 2.0 is more...