Thunderbird doesn't have a server. Email providers have servers, but they only store your messages if you set them to do so. If the account used a POP connection, then in all likelihood your messages were stored only on your computer and are effectively destroyed now.

Reformatting a hard disk is a serious operation and should not be undertaken until you are positive and confident that you have the necessary backup to restore data you want to keep. In this case you do seem to have made an unwarranted assumption that someone else was looking after your data for you.