Well, first things first. Do you have backups of the device (backups that are OUTSIDE the device, I mean), and if not, do you have anything of value on the phone that you would need to back up?

If you have backups, or do not care about the data in the device, full flashing it (rootfs+emmc) will recover the device to "factory state"

If you have data on the device that you need to back up, then it gets a bit trickier but it is doable.