Root How can I recover my Nexus 9, which booting (swirling dots) indefinitely?

[InterMurph]

My Nexus 9 (non-LTE) displays the swirling dots indefinitely. I have tried to fix it in several ways, but I have failed; it still boots indefinitely.

I have tried Power button/down arrow, then "FACTORY RESET". No luck.

I have tried Power button/down arrow, then "RECOVERY", then Power button/up arrow, then wiping cache partition, and wiping compeltely. No luck.

I have downoaded the Nexus Root Toolkit v2.0.5 from WugFresh. I have tried Power button/down arrow, then "FASTBOOT", then "Flash Stock + Unroot" from the toolkit. No luck. It says:

writing 'bootloader'...
(bootloader) Device State : Locked
FAILED (remote: Can not flash any images while device state is locked!)
finished. total time: 0.381s

The device does in fact say "*** Device State - LOCKED ***" at the top of the screen. When I try "Unlock Bootloader" from the toolkit, it connects, then reboots the device, but it then returns to its indefinite boot/swirly dots mode. When I power it down, then Power button/down arrow, it still reports "*** Device State - LOCKED ***".

I don't need to recover anything from the device. I just need it to boot up again so that I can start over.

What is my best approach?

Thanks.