NEED HELP!! My nexus is on some kind of restart loop!!

My nexus suddenly reboot and keep doing the same thing every time, i can put my pin and all but after 1 min of put the pin my phone reboot and do the same thing over and over. I try to flash a new ROM but even after doing that my phone restore my previous ROM and even the files and settings. I try to change the recovery to TWRP and after the restart my recovery change to my old recovery(cwm) i dont know what else to do. I try everything, flash, wipe, unroot,root, restore to factory rom and nothing. PLS I REALLY NEED HELP!! Thank you.