Did you perform or get an auto update? If yes, perhaps your firewall/antivirus program needs to be reset to allow Thunderbird. Example: I use Norton Anti-Virus/Firewall. Sometimes, after an update, Norton still thinks it can allow only the previous version. I then have to close Thunderbird and access Norton. Settings > Network > Smart Firewall > Program Rules Locate Thunderbird and set to 'Allow'. On one occassion, I have had to block, ok and then reset to Allow. If you use another Firewall/Anti-virus program, then there will be similar settings. So check this first.