Tried to go back to stock rom and phone keeps rebooting

Hey, I was recently using the Nexus Experience ROM and ran into issues with my mobile data not working. So, I decided to go back to 4.4.4 and using TWRP I ran a backup from August and it asked me if I wanted install superSU and I installed that and it rebooted and its stuck on Preparing device. its loops from the motorola boot screen to preparing device over and over. I think ive bricked the phone. any help on how I can fix this. thanks