What error messages are you seeing? Server settings?

no messages. it just does"t connect monday. it connected today, tuesday, & got emails ok. every once & a while this happens. connects ok for quite a while then not. a day later it connects ok. my web emails work just fine all the time. it is just thunderbird.

This sounds like a network problem, not a Thunderbird problem. You might try help>troubleshoot mode to gather data on the problem to share with your email host.

not a network problem since the net is fine for everything else. even yahoo/frontier web mail is fine.

• Please start *Windows* in safe mode with networking enabled

 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

• Still in Windows safe mode, start Thunderbird in Troubleshoot mode

https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

Does problem go away?

- If no, then cause is either: bug in Thunderbird, something (eg a setting, file or folder) in your Thunderbird profile, your mail provider. Please post into topic the contents of Help | Troubleshooting | copy text to clipboard

- If yes, the problem is gone, then (still in Windows safe mode) ... start Thunderbird normally -- If problem is still gone, then cause is a program loaded during windows startup. Possibilities include: antivirus SW, virus/malware, background downloads such as program updates -- If problem came back, then cause is likely a Thunderbird add-on - eliminate them by disabling add-ons one at a time in Tools | add-ons | extensions and restarting (or if you have many, disable multiple add-ons at a time)

- If results are unclear ... possibilities include temporary conditions such as contention from other running programs, downloads related to windows update, ...

Please let us know your findings.