The onboard "on" led on the Arduino Uno board lights briefly upon connecting to USB then fades out. Programs cannot be uploaded and the "Serial Port" item in the menu is grayed out. Does this mean the board is toast? Thanks!
It sounds like your board has a short circuit from +5vdc to ground and the USB thermofuse is opening up after a brief period to protect your PC's USB port. Do you have anything wired to the board or any shields attached? If so remove everything and try again. If the problem is still there then your board is toast.