If it is not going back on when you plug the USB back in then it is possible that you fried it. OR you have a possible short somewhere. Remove all your wire and try the USB again.
I can go back and reinstall the program and it will start to run again.
If I remove the 9 volt battery and reinstall the USB cable, hit the restart button the program will not start.
Right now I have it running the blink sketch with the USB cable but if I remove the USB cable and REconnect it the blink sketch will not start.