Computer crashes on login, greeted with flashing folder icon
It may be as simple as the cable has become loose.  If that is the case, it is a fairly easy fix.  Sorry to see you don’t have a backup.  If the drive has gone bad, that would have been your recourse.

For future reference, a simple external drive connected via USB will work fine as a Time Machine backup disk.  Even if you are not as regular in connecting it as you should, you would at least have a backup in case of catastrophic failure.  There are other online backup solutions too that are subscription based.

If you are comfortable with taking the MacBook apart, you can check the cables first.