Follow whatever rainisto says - it'll wipe all your data off the N9, clean.

If the N9 doesn't respond to a USB cable, start Flasher, let it wait for device, then plug in the phone (assuming you're starting with the N9 off).

If you're in a mad hurry, start the erase procedure, wait 1-2 minutes, then close Flasher and force the N9 off via the power button.

Flash main and emmc images as usual, without the erase-user-data flag.