Nexus 10 not booting, showing Google and lock icon when turned on This is odd. My Nexus 10 has been working great a quite a while now, but now, suddenly, when I turn it on, its quickly shows a Google title in the middle, and a lock icon at the bottom (in landscape mode only). The lock looks like a padlock and it looks open (not fully closed). Nothing happens when I wait. I can turn the tablet off by holding the power button for about 5 seconds. The tablet then stays off, until I press the power button for about 1 second. "Google" and the open padlock icon then reappears.
I've image-googled for anything about not booting and lock icons and whatnot, but I can't find anything that resembles this.
I've had an ipod touch years back and after my toddler tried to log in a few times, I lost access to it, and lost all my data. This made me not wanting to try Apple again. I seriously hope I'm not dealing with something like this here now again...
So if the lock icon was closed before and just opened somehow, you should find out what happens.

Now the lock is opened, so your boot-loader is opened. Have you opened it?

There are BL-unlocking apps available but all of them need root permission so I think this is not the cause. But I'm sure the device did not open its BL.


  I lost access to it, and lost all my data. This made me not wanting to try Apple again. I seriously hope I'm not dealing with something like this here now again...


On the Nexus devices if you open the device's Bootloader all data will be deleted. It's a security concern. The Android documentation doesn't provide a lot of elaboration, stating simply:


  The procedure must be confirmed on-screen, and deletes the user data
  for privacy reasons.


Once the bootloader is unlocked, the rooting process is quite easy so all of the information on /data essentially becomes accessible. This could be emails, contact information, or many other pieces of personal information that you likely don't want in the wrong hands. Therefore, it makes sense to wipe all of this data during the unlocking procedure.

TL;DR: If the device bootloader was opened somehow, all of your data will be gone.