Dear All
I have been using TB for years and for months its playing up. The host for my email have looked into it but cant help. Any ideas or what other program can I use instead?
I keep getting this when trying to send:
I think this tells you how to remove the Master Password if you don’t know what it is.
Thank you!
I did have a look at this but made no progress 
As nivaq said, it’s probably a master password issue.
More details from nivaq’s link…
There is a known bug that sometimes occurs when updating from 2.0.0.24 to Thunderbird 3.* that causes Thunderbird to think you set a master password even though you never created one. If this happens, try resetting the master password as explained above. If that doesn’t work exit Thunderbird and delete the key3.db file in your profile.
This caught me out awhile back, I fixed it as above, by switching off the master password.