Reload operating system

If you need help

Learn what to do if you can't start up from macOS Recovery.

If the question mark isn't persistent, but appears only briefly before your Mac starts up, make sure that your startup disk is selected in Startup Disk settings. If that doesn't help, reset NVRAM and check the setting again. 

If you still need help, please contact Apple Support.

The information in below Apple Support Document should provide the need information

If your Mac starts up to a question mark - Apple Support

Command R doesn't work....I still get the flashing question mark after about 15-20 seconds. What now?