Sometimes this is a BIOS issue.
Reboot your computer and hit the "Delete" button, during "post".
It should be a black screen with white alpha numerics. At the bottom of the screen it may tell you to hit a different key, like F2.
In the BIOS,check your usb setting, in case it has gone haywire.
Also update the info in the "My System Specs". More information makes it easier to trouble shoot.