[Google Pixel XL Questions & Answers] SirRGB:
Sorry I cant find where I got this PDF. Just download https://developers.google.com/android/ota <----the one for your phone then do below.

I had the same problem. I did below and its all back to normal. For ME to apply OTA I had to fastboot then press up volume till it said recovery. Press power then hold power down press up voluem once and there you see where you want to me. SIDELOAD. Then you can do the OTA img

4. Boot the Pixel into Fastboot Mode. This can be done by first powering off the Pixel, then pressing and holding both Power and Volume Down simultaneously. You should see a green android on his back and Start in green with an arrow pointing toward the Power button. 5. At this point do a test to see if drivers are properly installed in the PC. Attach the USB cable between the PC and the Pixel. Device Manager should now show an entry of Android Phone with a sub-entry of Android Bootloader Interface. 6. In the command prompt on the PC from step 2, enter the command fastboot devices. If you see a response with the serial number of your Pixel, then all is good and proceed. If the results for step 5 or 6 are not as expected, then further work is required in the PC to get the required drivers installed, so do not proceed further and ask for assistance. 7. Remove the USB cable between the PC and the Pixel. 8. Tap Volume Down until you see “Recovery Mode” in the arrow, then tap Power once to select it. 9. After a few seconds you will see the Google logo followed by a green android on his back with “No command” underneath. Press and hold Power, then tap Volume Up once, then release Power. NOTE: If your device starts rebooting at this point, your only recourse is to contact your place of purchase about a warranty replacement or repair. 10. You are now on the Android Recovery screen. On the third line at the top of the screen you will see something that looks like this: 7.1 / NDE63V / 3389651 The middle one is your current Build Number. Make note of this as the Rescue OTA you need corresponds to this one or a more recent one. (NOTE: You cannot install an older one than your current Build Number.) 11. Tap Volume Down until the entry “Apply Update from ADB” is highlighted, then tap Power to select it. You Pixel is now waiting to receive the OTA file. 12. Attach the USB cable between the PC and the Pixel. Device Manager should now show an entry of Android Phone with a sub-entry of Android ADB Interface.

13. In the command prompt on the PC from step 2, enter the command adb devices. If you see a response with the serial number of your Pixel, then all is good and proceed. If you do not see this then further work is required in the PC to get the required drivers installed, so do not proceed further and ask for assistance. On a Windows 10 PC: 14. On your PC, create the folder c:\MyPixel 15. Download the OTA for the current Build Number of your Pixel from here. Note: There are currently three release streams for Pixel phones. Please ensure you pick the correct one based on your situation: Release Global North America Verizon Oct 2016 NDE63H NDE63L NDE63P Nov 2016 NDE63U NDE63V NDE63X

16. When the download finishes, click on “Open Folder” at the bottom of the browser window. This will open Windows File Explorer. Right click on the highlighted file that was just downloaded, click Cut, then select the folder c:\MyPixel, right click again and click Paste to move the OTA zip file into c:\MyPixel.

17. If the Pixel Apply Update from ADB timed out (step 12), then re-initiate it. 18. Enter a command similar to the following (this example is for non-Verizon, North American customers), but the highlighted filename will be the one you pasted into c:\MyPixel in step 16 (don’t forget the .zip extension in the command): For a Pixel: adb sideload c:\MyPixel\sailfish-ota-nde63u-f1c2a8f5.zip For a Pixel XL: adb sideload c:\MyPixel\marlin-ota-nde63v-272853b5.zip 19. Progress messages should now appear on both the PC and the Pixel. Be patient as there can be several minutes before any progress messages appear. If there are any errors, do not proceed further and ask for assistance. On the Pixel: 20. When the update finishes the line “Reboot system now” should be highlighted. Tap power once to initiate a reboot. 21. After the reboot, your Pixel should be back in operation.

Thats what helped: The post of Zeblade.