I've only encountered something similar once--similar, in that the machine sometimes would boot and sometimes would not, but it wasn't Fedora--and that turned out to be a botched install regarding the directory structure on a GPT partition.
Have you tried looking in the log files?
https://docs.fedoraproject.org/en-US.../viewing-logs/
My suggestion would be that, the next time this happens, when you do finally get in, take a look at the most recent log entries surrounding the time at which the problem occurred. If the answer isn't readily apparent, you can post the relevant bits here, being sure to surround any terminal output with "code" tags which become available when you click the "Go Advanced" button beneath the compose post window.