The same thing happened to me. I've had my Nexus 7 for about one year. Something happened that completely drained the battery. I charged it and now when I turn it on, it prompts me to enter a password to decrypt storage. I've never set a password for this tablet. How do I start it in recovery mode in order to wipe it out and completely start over? I can't get to any other screen except this keyboard screen to enter my password.