I am not receiving emails (but am able to send them).

Hi - Thunderbird has not fetched any emails from the past two days. I am running 153.1.0esr (64-bit) on Windows 10. If I remember correctly, I downloaded that version four days ago. I vaguely recall getting some message somehow that I needed the ESR. (Sorry I don't have a very good memory!) Thunderbird did fetch emails for two days after that...but I suppose it's possible that I didn't restart Tbird right away, so maybe it was still using an older version until two days ago?? My ISP is RCN/Astound. Last week, I changed my password on both the RCN webmail and on Thunderbird, and I could send and receive after that. Tonight, when I discovered that Thunderbird wasn't fetching, I also tried to send an email, but I initially received an error message. It was something like "Your message was unable to be saved" and then there was something about saving it as a draft. I've seen that message many times over the years, but I don't remember exactly what it said. Thinking it might be a password requirements issue, I changed my password again. I don't know if that mattered, but I am now able to send emails -- but I still can't fetch them. I turned off my Windows firewall and Windows real-time threat protection. That didn't help. I saw this thread https://support.mozilla.org/en-US/questions/1531863 tonight and set "network.dns.disableIPv6" to "true." That didn't help either. I would appreciate any assistance you can provide. Though I am not a expert, I am happy to provide more info if needed. Thank you in advance for your help.