To unlock your Nokia Lumia 900, you need to hard reset. This is by pressing and holding the volume camera keys at once. After this you will have to hold the power button until the logo screen goes black. After the screen is black you can then press the volume and camera keys again until rebooting takes place. The phone will then turn on without asking for a password. If this does not work, it will be important if you take it to a technician.