How do I restart my late 2013 Mac Pro after failed installation of high Sierra

The screen with a folder/question mark would indicate that your Mac Pro cannot locate a start-up volume. Of course, this may be due to an incomplete or failed upgrade attempt. However, it may also indicate a failing hard drive.

Please try the following, in order. Stop at any step where the issue is resolved:

1. Reset your Mac's NVRAM or PRAM.
   • How to reset NVRAM or PRAM on your Mac - Apple Support
   • Resets sound volume, display resolution, startup-disk selection, time zone, & kernel panic information.
   • <option><command><P><R>

2. Run Apple Diagnostics to uncover any hardware issues with your Mac.

   • How to use Apple Diagnostics on your Mac - Apple Support
   • Apple Diagnostics: Reference Codes - Apple Support
3. Boot up your Mac in Recovery or Internet Recovery Mode to verify the startup drive using Disk Utility's First Aid option.
   • About macOS Recovery - Apple Support
   • When at the initial Recovery Mode screen, select the Disk Utility option.
   • Run Disk Utility > First Aid to verify the startup disk's integrity.
4. Boot up your Mac in Single-User Mode to repair the startup drive.
   • Start up your Mac in single-user mode or verbose mode - Apple Support
   • Resolve startup issues and perform disk maintenance with Disk Utility and fsck - Apple Support
   • How Can I Repair My Hard Drive If My Mac Won't Start? - Lifewire
   • At the command prompt, type: /sbin/fsck -fy <return>
     • If fsck makes any repairs, you will get a message that says: Files System was modified. If you do, run fsck again until the "The volume <volumename> appears to be OK."
     • Exit messages:
       • Issues found = "File system was modified"
       • No issues found = "The volume <name> appears to be OK"
   • Type reboot at the prompt to exit Single-User Mode.
5. Boot up your Mac in Recovery or Internet Recovery Mode to re-install macOS.

   When at the initial Recovery Mode screen, select the Reinstall macOS option.

6. Make a Genius Bar appointment at your local Apple Store.
   • Check Your Service and Support Coverage - Apple Support
   • For getting the right support, here's a genius idea - Apple
   • To contact Apple Support directly.
   • You can only make 1-week ahead appointments either via the web link above or AppleCare phone support. The appointment timeframe is longer if you go in-person to an Apple Store.

Uh oh. What your Mac is telling you is that it can’t find a valid system folder to start up from. .so-reset the smc/pmu by disconnecting the power cord... wait about 30 sec. plug it in. Plug in a usb keyboard. Press cloverleaf/command key + r together. Hold down till you get to recovery partition. Move mouse to top center of screen to os x tools. Run disc utility. See if you can reinstall os. Choose main disc as startup disc. Reboot mac. For the record the high sierra installer is pretty fussy. Needs to install a firmware update and check for a mac capable video card. If either of these fails, then high sierra will not install

Good luck

John B

Just a little additional info: My high sierra install attempt on a 2015 MBA appeared ok until I shut it down. After that, I got no response whatsoever and it would not connect to recovery, nor would it recognize any external drive. After many attempts, it finally recognized my external clone and that was the only way to get it back into working order.

If you have an external bootable clone, try that route. And, make sure you use the correct key combo for recovery as there are several different ones, the link has already been posted.