start up

This indicates the computer could not locate a valid system file to boot from. First, restart the computer and hold the Command and R keys until you see the Apple icon. If this fails try again holding the Command, Option, and R keys. (For this second method you’ll need to be able to access the internet via WiFi.) Once the computer has finished booting you’ll see a menu. Select the Disk Utilities option.

In the left window pane at the very top you should see Internal and under it you should see either Apple SSD or the name of your hard drive (Macintosh HD if you’ve never changed it.) If you see neither of those your problem is that the computer can’t find its hard drive. You’ll want to make an appointment at your local Apple store or other Apple authorized reseller. Otherwise, select the hard drive icon (the top icon if you don’t see Apple SSD, the second icon if you do) and at the top of the window look for the Disk First Aid button and select it.

If no errors were found or if errors were found but are reported as fixed, quit Disk Utility. If errors were found and weren’t fixable you’ll need to erase the hard drive and restore from a backup. If you have no backup you may be able to retrieve data from the hard drive but we’ll leave that for another time. To erase the drive select the Erase button at the top of the window then quit Disk Utility and from the menu select Restore from Time Machine.

If no errors were reported you have 2 choices. Probably you can download the Mac OS and get back to work or you can Restore from Time Machine. In either case, quit Disk Utility and select your choice form the menu.