Mac computer 2012

Hi there!

Before you saw the flashing question mark, did you erase your Mac using Disk Utility? If so, you'll need to reinstall macOS, and check that your disk is setup for macOS to be installed. See if you can follow these steps:

1. Start up from macOS Recovery: Turn on your Mac, then immediately press and hold these three keys until you see an Apple logo or other image: Option, Command (⌘) and R. 
2. From the utilities window, select Disk Utility and click Continue.
3. Select View > Show All Devices in Disk Utility.
4. Select the top-level, non-indented, disk in Disk Utility-it’s at the top of the hierarchy.
5. Click the Erase button in the toolbar (if Erase is grayed out, select the entry just below it), then enter the requested details:
   1. Name: Macintosh HD
   2. Format: APFS or Mac OS Extended (Journaled), as recommended by Disk Utility
   3. Scheme (if shown): GUID Partition Map
6. Click Erase Volume Group. If this button isn't shown, click Erase instead.
7. When done, quit Disk Utility to return to the utilities window.
8. Select Reinstall macOS in the utilities window, then click Continue and follow the onscreen instructions.

Hope this helps! If you have any questions, let us know!

Jack

That means the startup disk is no longer available:If your Mac starts up to a question mark - Apple Support

Note: the last instruction is, "If you still need help, please contact Apple Support."