Why is this happening, and how can I get past it?
MacBook Pro 13", macOS 10.14
follow the ref: About the screens your Mac displays as it starts up - Apple Support
Prohibitory Sign:
A circle with a line or slash through it means that the selected startup disk contains a Mac operating system, but it's not a macOS that your Mac can use. You should reinstall macOS on that disk.
The first question I would ask is this the OEM hard drive that shipped with the Mac new? A seven yr old HD would be suspect.
You can easily check the health of your hard drive with DriveDX. If you have a third party SSD installed, then please post a screenshot of the "Health Indicators" as DriveDX doesn't work as well with SSDs. Feel free to post a screenshot even if DriveDX reports a drive as healthy, since those indicators may warn of a worn out drive.
If you have the MBPro 13" (mid-2012), then it could be just a bad hard drive cable. The MBPro 13" (mid-2012) hard drive cable had a very high rate of failure in this particular model. The best way to know if this is the case is to remove the drive and boot it externally (after first verifying the health of the drive).
The Apple Hardware Diagnostic may detect a failing drive, but DriveDX is more likely to notice an issue with a drive.
Thanks, Leroy! What might cause this problem? I was using my MacBook pro just yesterday with the same OS.
Also, what should I do to reinstall an OS that my Mac can use?
The first question I would ask is this the OEM hard drive that shipped with the Mac new? A seven yr old HD would be suspect.
You can run this utility http://etrecheck.com to get a good look at your System config. to see if anything else jumps out.
If you need help interpreting the output you can post the report in its entirety here in Additional txt box:
my 2012 macbook pro shows this 🚫 when i try to reboot 🚫. How do i get past this