My Mac Pro screen flashing With a ? An below states support.Apple.com/Mac/startup
My Mac Pro screen flashing a ? Mark , an below states support.Apple.com/Mac/startup
MacBook Pro 13″, macOS 10.12
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That is the url to which you should refer, which links to this article:
If your Mac doesn't start up all the way
https://support.apple.com/en-us/HT204156?cid=mc-ols-mac-article_ht204156-macos_ui-10062020
It shows the several different Icons you can get, including flashing question mark.
Flashing question mark means, "Could not find a bootable Volume at the location last specified as 'Startup Disk'."
Step 1 is to use Disk Utility and attempt to repair that Volume from Recovery mode.