Option 1, if the device is not fully blocked and if you remember the valid Admin PIN use Nitrokey App to unblock the Nitrokey. Your keys won't be lost:
Open a terminal (on Windows: Press the Start button and enter "cmd") and start the Nitrokey App with "nitrokey-app --admin". Klick on the Nitrokey App's tray icon, select "Configuration" and "factory settings".
Option 2, to reset a blocked device, Windows only:
Option 3, to reset a blocked device with GnuPG 2 and Windows:
Option 4, to reset a blocked device with GnuPG 2 and Linux:
Option 5, to reset a blocked device with GnuPG 2.1:
Option 6, to reset a blocked device using OpenSC:
Install OpenSC and execute "openpgp-tool --erase" in a terminal.
You can find out the version of your device by executing gpg --card-status
In order to reset a Nitrokey Start 1.0, you need to define a public key for firmware updates beforehand! In case of a blocked device it enables your to perform a firmware update which resets the device.
You may also define a reset code which enables the reset of the User PIN (not Admin PIN).
As long as you know the unblocked SO-PIN you can initialize the device as described here. There is no way of resetting the Nitrokey HSM if the SO-PIN is forgotten or entered wrongly 15 times. In such case the device can't be used anymore.