I rooted my OnePlus 3 a while ago and to do that you must unlock the bootloader which went smoothly; this was a year ago. Now it is owned by my sister who wanted it locked so when I typed in the command into cmd the phone restarted and then got to the oneplus logo, vibrated then restarted, then went to the oneplus logo, vibrated then restarted over and over. I have managed to get to the fastboot menu but I cant find a way to get past the wall of restarts.