Nexus 6P Signal comes on and off.

I was on cm13 and my signal suddenly disappeared. I flashed stock but it did not get fixed. Imei Number is visible.
Please help,.

'Power off' button on lockscreen, doesn't let me do anything else.

Gave my Z2 for repair at a local shop and they replaced the battery and reset the firmware to an older one - 17.A.2.0.25 (i think) running Kitkat. On the lockscreen there's a giant Power off button that doesn't go away, and I've traced it to com.sonyericsson.startupflagservice. I rooted and upgraded to Marshmallow, and now the lockscreen stays there and keeps popping back, preventing one from using the phone for anything. I've heard suggestions to use ES file explorer to delete this process, but I cant do anything with the damn screen getting in the way. Is there any other way to fix this?