My name is David.
This issue often occurs when there is storage disk corruption that needs to be fixed so your Mac can access the storage drive again.
To resolve the issue power down the Mac completely by holding down the power button until it powers off. Then hold down the Command and R keys, and power the Mac back up again. Keep pressing Command+R while your Mac is booting up.
Once your Mac starts in Recovery Mode you will gain access to the Utilities screen. Click on Disk Utility, locate the icon for your Mac's drive (most likely there is just one disk), and click Verify Disk.
If there are errors with your disk Disk Utility should find them and ask if you would like to repair them. Click Repair Disk to fix the disk errors.
One the fixes are completed, exit the utility, and restart your Mac to test and verify that the boot process is fixed.
Please let me know the result and if you need further assistance. Also, please provide a positive rating (3, 4, or 5 star) as that is how I receive credit for my work.