how to fix the file system?

[Guy Abandon.]

If it's corrupt, it will get scanned automatically I think, Slackware
9 here does.  Then you do get the opportunity to run fsck which is the
File System ChecK tool.  This can take a while and take several passes
to get it right.

Yes, I got caught on this too, can't remember how.

If you get back on long enough to do real backup, a rebuild has its
uses as you never know which bit got corrupt and is there like a time
bomb waiting to blow you away when you do something different to
normal.

Patching any kind of file allocation table is a chancy affair leading
to the loss of a complete or significant part of a file.  It takes a
lot less than a whole allocation block out of a binary to wreck it and
make its operation very risky and unpredictable.

I don't know the low level workings of Linux to know if it has
duplicates of allocation tables, even DOS used to though don't know
about the Winglows.

GA!