To restore a nandroid backup, the device cannot be in "normal operation mode". You have to boot into your (custom) recovery (see our tag-wikis on: recovery-mode / clockworkmod / twrp), from where you can trigger a full restore. With ClockworkMod Recovery, it is e.g. found in the Backup & Restore sub-menu.
Stock recoveries don't offer taking/restoring Nandroid backups. But as you were able to create one, it's pretty safe to assume you've also got a custom recovery installed to your device.
