What's worked for me a few times: Find a few messages
you no longer need, and delete them. Then restart Windows
Mail, after shutting it down WITHOUT doing a compaction.
That brought back many folders that were only hidden, not
fully deleted.
Also, if you've accidentally deleted something but still have
the Deleted Items folder, look for it in that folder.
Some types of failures move messages to a Recovered
Messages folder, creating that folder if needed, so check
there as well if that folder is visible.
I have found, though, that if you let too many messages
accumulate in the same folder (well over 50,000),
expect at least some of them to eventually disappear in a
way that's harder to bring back, if it's even possible.
Moving some of the messages to storage folders, and
keeping those below 50,000 messages each. will avoid
this.
Do you do backups? Restoring from a backup can be
safer than a system restore.
Robert Miles