[Nexus 9 Q&A, Help & Troubleshooting] GedBlake:
Hi, conceptics...

If you haven't unlocked the bootloader, or enabled the 'OEM unlock' setting in Developer Options...

SETTINGS>> DEVELOPER OPTIONS>>OEM unlocking (allow the bootloader to be unlocked)

...then your options are limited.

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With an unlocked bootloader, or at the very least,

with the option to unlock the bootloader

, (and which requires the above setting to be enabled, and which requires Android to be fully booted in order to enable it), your Nexus 9 would almost certainly be recoverable by simply reflashing it with a Nexus 9 Google factory stock image, or fastboot flashing TWRP Custom Recovery, and then using an ADB 'pull' command to copy your stuff off of the device to your laptop/PC.

But with a locked bootloader, and likely an unlockable bootloader, these options aren't available.

I suspect you're going to have to return your device under warranty :(.

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But before you do...

Have you tried booting the Nexus 9 into Safe Mode?

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Safe Mode is ostensibly an Android troubleshooting mode, designed to temporarily suspend all third party (possibly rogue, badly written) user installed apps (those installed via PlayStore or user 'sideloaded' APKs) and just boots the bare essentials of the Android OS.

Now normally, booting into Safe Mode is accomplished by pressing the Power button as normal for shutdown, and then LONG PRESSING on the 'Power Off' dialogue box/option that appears and which you then just normally tap on.

(See second and third attached screenshots.)

Obviously this isn't an option if Android won't boot in the first place, due to it bootlooping.

But like the older Nexus 7, the Nexus 9 can be Safe Mode booted from a complete shutdown state.

Here are the instructions...

Full post here:

http://xdaforums.com/showthread.php?t=2133986

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Precision is required for this button combination...

Power on the device, and RELEASE the power button after the black and white Google logo has displayed for about 2 seconds, then PRESS AND HOLD the Volume Down button ONLY... and KEEP IT HELD DOWN until the device completes booting.

Whilst this Safe Mode boot instruction is written with the Nexus 7(2012) in mind, it also works with the Nexus 9.

I've just tested this procedure on my own Nexus 9. See last attached screenshot

As you can see from the screenshot, all of my user installed apps are 'greyed out' and are temporarily unavailable for use. Notice also the Safe Mode message in the lower left hand corner.

(See attached screenshot four.)

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If you do successfully boot into Safe Mode, you can then uninstall any or all third party apps that might be problematic, and more importantly go to...

SETTINGS>>ABOUT TABLET>>Build Number ...and then tap 7 times on the Build Number until you become a 'Developer' (...if only it was that easy... :) )

...and then go to...

SETTINGS>> DEVELOPER OPTIONS>>OEM unlocking (allow the bootloader to be unlocked)

...and enable (checkbox tick it) that infernal setting that has caused so much trouble for Nexus 9 owners!!!

(See first attached screenshot.)

If all of this works, you can then unlock the bootloader, and reflash the device... :).

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My suggestions here are what in America I think are called a 'Hail Mary Pass', and I am not particularly optimistic it will fix your Nexus 9 :(.

But it's gotta be worth a shot.

You have nothing to lose by trying it.

Best of luck, and I hope this helps.

Rgrds,

Ged.