It usually means your Master Password.

Is that right smartarse? I don't have a master password dickhead that's why I asked for help !!!

Why didn't you say so?

Many users complain that the master password doesn't do anything. They don't enter it, press "cancel" and they are let in to Thunderbird. If this is so, then I would hope that this password prompt isn't actually stopping you from getting in.

Do you allow Thunderbird to store your passwords? If so, they can be found under Tools|Options|Security|Passwords→Saved Passwords - but you may well hit the master password prompt again, notably if you try to view stored passwords. .

Has the "use a master password" box ticked itself? Does unticking it fix things?

Do you know all your passwords and logins? Would a procedure that resets them all (in order to remove a Master Password) be of use to you?

No-one can help you if you don't describe what is (and is not) happening. Insults don't help anyone.