Power the arduino (just the arduino, remove everything else that might be plugged into it) by connecting an external source(7-12v) to the barrel jack. Does it turn on? If it does, what is the voltage at the 5v pin?
I tried doing it but unfortunately my computer doesn't recognize the Arduino...
make sure you have the right drivers installed. You may even try reinstalling the IDE software all-together. If that doesn't work, look into DFU mode for the USB chip. You might try simply resetting it before re-uploading the firmware.