My IMAP access has suddenly stopped working. I can still send messages by SMTP and view my email online, but I cannot access IMAP.  Thunderbird gives me the error "account is blocked."

I have double-checked all server settings; nothing has been changed.  I cannot access IMAP on either my phone or PC.  Has my account been flagged and blocked for some reason, or has there been changes made to the IMAP setup in the past week?