Reply to s4 power issues on mac - URGENT HELP NEEDED

Hey guys,
I played a gig on the 27th and i am suppose to be playing again this evening but i took my s4 out to have a practice before hand and now it appears that it doesn't seem to be receiving any mains power and when i try to run it off purely the usb the mac displays an error message that says a usb device is trying to use to much power from the mac and it has been disabled. Does anyone know how i fix this because at the moment i am pretty much screwed!!!!