Last time I saw that the cause was Nortons antivirus. What product are you using that is probably failing.

I have tried the two suggestions offered but I still cannot download any incoming nor can I send any emails. Using file manager I looked at the active profile and mail sub directory and the "date modified" shows updates since the 24th of March to current but non of the files since Mar 24 are displayed in the Thunderbird menu. Accordingly the number of files do not reflect any of the missing emails in the inbox etc. It looks like the email files are in my computer but remain hidden ???? I cancelled all filters so the mystery continues . Is it a coincidence that my problem started around the same date that VER 149 was released ?

Coincidence.

Something in my system was corrupted . I tried many solutions without success. Finally I renamed my files , removed all Thunderbird program(s) . After I installed a a new download of Thunderbird (VER 149) I created a new TB database and copied my old data , emails, addresses etc I now have a stable TB file. Sometime a man got to do what a man is got to do......... you can quote me on that .

@RKimak, Thanks for posting your results.