http://kb.mozillazine.org/Recovering_a_profile_that_suddenly_disappeared

Thanks, have done that. Pity though that one's profile can just disappear, and this obviously happens more than once given the elaborate help file. Maybe a point to improve on for Thunderbird?

Profiles don't disappear, but Thunderbird looses them. Once is a very long time. Cause unknown.

[Profile]
Name=Patricia
IsRelative=1
Path=Profiles/o4t15t9h.default

Is the sort of data in the profiles ini fiole that get messed up so Thunderbird can not find it's profile. In some cases it is the profile.ini file that is actually missing. Nothing else.

The elaborate article is probably 10 years old and been having minor tweaks all that time.

Please go to your start menu. and type %appdata%\thunderbird\profiles into the search box and press enter.

what folders appear in windows explorer?

The profile.ini is OK. It points to my mail in G:\email. It must be that the prefs.js file there was garbled. The G-disk is on a NAS. Maybe putting windows in sleep mode with Thunderbird running caused the problem.

There are probably a hundred reasons why putting production data from desktop applications on a NAS is wrong. Not the least of which is that unless you have a UPS on both then a power outage will affect the application and NAS differently.

I am also uessing that your main profile is still on the C drive, so Thunderbird when it could not access the NAS simply lost the [pointers to those files as the NAS was inaccessible.

Only profiles.ini is on C:\ All other files are on G:\, including my prefs.js But I can move that and all other files except for the mail itself to C:\ (which is a relatively small SSD). Perhaps that will give me a more robust installation.