so it's best to go through step-by-step recovery before assuming the wrost-case scenario occured.
Running the suggested fsck/filesystem checker (in checking only mode) is simply the first thing to do with a misbehaving filesystem.
The rest depends on the results but isn't exactly long either.
If it finds no errors it probably isn't the filesystem. Look for problems elsewhere.
If it finds unfixable errors there's not much choice too: wipe it (create a new filesystem or restore a backup from before the problem).
Only if it finds fixable errors there are some other steps:
The very first thing is making sure it isn't caused by defect HW.
If it is, fix it. (Included for completeness, I doubt its the cause here)
Then optionally make a backup of the complete FS.
The next thing is telling the checker to fix the error. Repeat as needed. Possibly with different repair tools.
If you run into problems you can restore the backup from the step before to try again.
If you run out of pacience: wipe it
And that's about everything you can do.