It's often possible to restore a profile by clicking help>troubleshootinginformation, scroll to 'profiles' and click 'about:profiles' - this opens a window that lists profiles. If your recent one is listed, you can activate it there. If not, there is a 'create profile' button there that will let you locate the profile and then activate it. If you have thunderbird set for automatic updates, they come unannounced , so you may prefer to change the setting. VPNs are a known concern, but not a strength of mine, so I cannot comment there.

The Thunderbird profile that contains my historic email, when I try to import it, generates an error message of:

"Import failed unexpectedly, more information may be available in the Error Console."

Do you have any thoughts on what the problem may be?

Was the profile zipped? If so, you might try unzipping and using tools>import>import from another tunderbird installation. No, I have no idea on the problem, but if the profile is unzipped, it can be copied in with windows file explorer. OR, if it's unzipped and somewhere on your hard drive, you can activate it by selecting help>troubleshootinginformation, scroll down to 'profiles' , click 'about profiles' and click the 'create profile button, click next, assign a name, click the browse button to locate and select your profile.