Cannot startup my mac

Try to hold option at start up to see if it finds a start up disk and if not then try to boot up in safe mode but after following resource’s below if your Mac is still unable to start up or find the startup disk you could run a diagnostics and see if it detects any hardware issues with the disk or contact Apple support.

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If your Mac doesn't start up all the way

https://support.apple.com/en-us/HT201232

If your Mac starts up to a question mark

https://support.apple.com/en-us/HT204323

How to use safe mode on your Mac

https://support.apple.com/en-us/HT201262

Use Apple Diagnostics to test your Mac

https://support.apple.com/en-us/HT201232

Contact Apple for support and service

https://support.apple.com/en-us/HT201232

lingcheng Said:

"Cannot startup my MAC: Trying to reinstall OS, but cannot find the disk to install"

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Mount the Hard Drive in Disk Utility:

While booted into Recovery Mode [Hold Down: command + R keys upon boot], go to Disk Utility> tap the Hard Drive, then tap the Mount button. See if you can then reinstall it. If it is already mounted, then unmount it and then remount it. See where you can go from there.