macbook pro can not see a startup disk when trying to start computer?

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If your Mac doesn't start up all the way - Apple Support

FYI, in the future it is best to post the exact model of your Mac and the version of macOS which is installed. Plus a little more detail on what you see when attempting to start the computer as well as any steps you may have tried to solve the problem.

Writing an effective Apple Support Communities question - Apple Community