I found the problem, In the View Mode - Current View, the selection "Hide
Read Messages" was selected. I changed that and it is fine, all the
disappeared messages reappeared. I discovered this because I did a search in
all folders in case they had bee misdirected. Good Tool!!
The big question to me is how did this happen?
I haven't been in the View tab in months.

Anyway, for me the issue is closed.