What security software do you use? This can be your issue. Try switching it off entirely (unless you are directly connected to the internet and don’t use a router) OR only disable the Mail protection component in that software and restart your computer, make sure the software is still disabled and retry. If that works, re-enable the security software, remove any rule and mentions of Thunderbird in it and re-add Thunderbird to the allowed programs. Also make sure that software is up to date, and consider leaving the mail component disabled - basically you don’t need it and it may cause issues. You may be able to find out what caused the issue to occur "suddenly". Did Thunderbird, your security software or Windows recently update?