What shows in the activity manager? (it is on the tools menu) What shows in the error console? (Ctrl+Shift+J)

Thank you, Matt. Luckily for me I have Hover as my provider and their phone help is awfully good. Before I got your generous reply I phoned them and in an hour's worth of work found that all my server parameters were wrong, both for the secondary account that had stopped syncing but also for the main account which had no problems. Hover help couldn't tell my how either one was working -- said they shouldn't be -- but directed me to correct the parameters. Somehow that didn't help, so finally they said remove both accounts and install them again (with correct parameters) and that did work, although it took most of an hour to reload all the emails for both accounts. By the way, the wrong parameters that had been working for both accounts for many months were the ones defaulted by Thunderbird, for example, mail.mann.org instead of mail.hover.com and wrong port numbers for both incoming and outgoing mail. So, in the end, I'm completely back up but I never found out how I could have been running for so long with all the wrong parameters. Thank you again for your help. The problem is solved and I'll see if I can find a button to signal that. I'll also be glad to try your help with the activity manager and error console, which I knew nothing about. I really seek out useful information like that.