Do you have recovery installed already?
1)If so boot into that and mount ur sd car to usb cable.
2)If you don't then go to fastboot, connect it to your computer using a usb cable, then unlock your bootloader, flash recovery and download a custom rom and put it in to ur phone through the method 1. Use the link I provided before.
Guide to unlock,root and recovery.
I don't think it's possible to get your old rom back, so it might just be easier flashing a custom one. Generally better anyway.
All the best mate, tell us what happens. I had the same issue but was able to fix it.