MacBook pro
How does a startup disc become unavailable or an os disappear on Mac you’ve used for 1.5 yr
Question marked as
If you mean a blinking folder with a question mark, then the file system may be corrupt, the OS may have erased, or your drive is failing.
First thing I would do is run the Apple Diagnostics to see if it detects a failing drive (it will only detect some types of drive failures). There are other better ways to check the health of the drive, but it depends on whether you have a bootable macOS USB drive or whether you will need to use a Linux boot disk.
Here is an Apple article on what to do when you enter a blinking folder with a question mark: