Locked out of email

Earlier this week my  .net  email account was working perfectly well and then at around 10am on Monday (31/10) that changed for no apparent reason and I have been unable to send or receive emails since. Neither have I been able to do this via my email client (Thunderbird) nor via webmail (as I can't login). I have tried via the troubleshooting guide to reset the server settings and I have tried to change my password via My Account. In fact, I can't even get recognition that my .net email address exists via My Account (I log into that using an .co.uk user name, by the way). Can you offer any advice or ideas for resolving this, please? I know that there is at least one important email to which I can't get access.