Default [Q] Nexus 7 (2013) bricked - Macbook won't recognise its USB

I rooted my Nexus 7 ages ago and was using it the other day but the next morning it wouldn't turn on past the Google unlock screen. I can get into bootloader and I have terminal ready to erase boot etc BUT my mac just won't acknowledge that its there. Attached with USB etc but Android File Transfer can't find it. I keep getting the message "Please connect your Android device with a USB cable to get started." I have tried everything I can think of, please help me!!! Thanks