I've got the same problem on a recent OTA update on my Nexus 9. Unfortunately thes "Nexus Root Toolkit" app didn't help because my Nexus 9 was all stock, no root and the OEM unlock was disabled. The problem got me into a dead end. But I found a solution that helped me out and now my Nexus 9 is alive again. It's simple, but you need to follow the steps thoroughly.
Be advised that your data will be completely gone, but at least your tablet will live again.
- Ensure your tablet is fully charged and that you have the 'adb' available (here it's how you can find it http://developer.android.com/tools/help/adb.html)
- Download the a OTA image here http://developer.android.com/intl/zh-cn/preview/download-ota.html for your device, mine was Nexus 9 Wifi so i picked the Nexus 9 "volantis"
- Start your Nexus and You'll see an image of an Android robot with a red exclamation mark (you may also see "No command").
- Press & hold the Power button. While holding Power, press the Volume up button once. Then release the Power button.
- You will then get into the Recovery Mode screen
- I executed a Wipe data/Factory reset, not sure if it's mandatory for this procedure, but I did, it's up to you.
- With the device connected with your computer choose the option: Apply update from ADB
- Execute the following command: android_sdk_path/platform-tools adb sideload path_to_the_ota_image_you_downloaded_on_step_2.zip
- Wait until it gets completed, restart the device and that is it!
Good luck!
It worked well for me on the first shot, I hope it helps you too.