If this occured aqfter an update and you cannot send nor receive then it is likely your Anti-virus/firewall is blocking Thunderbird as it has not recognised the update as the same program. Close thunderbird and access the Firewall, which might be part of your Anti-Virus product. You need to make sure Thunderbird is allowed. I have Norton and on one occasion, I had to set to block, ok it and then access Firewall again to resset to allow. then restart thunderbird.