If you have the Problem, that your Phone is trapped in a reboot loop, try this steps to recover your OS.
- Right on begin of a (re)boot where the Nokia logo appears: Volume Down until the exclamation mark appears
- Press Volume Up once
- Press Volume Down once
- Press Power once
- Press Volume Down again
Then the Phone will reboot again and shortly after this, the gears will show up and a fresh installation is starting.
If you do a mistake in the button order, just start again with step 1.