If you have the Problem, that your Phone is trapped in a reboot loop, try this steps to recover your OS.
1) Right on begin of a (re)boot where the Nokia logo appears: Volume Down until the exclamation mark appears
2) Press Volume Up once
3) Press Volume Down once
4) Press Power once
5) 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.