I'm not going to take responsibility for telling you how to re-install Thunderbird but will give you this advise: If you've got e-mails or anything else on your computer that you value you should have a backup copy. Hard disks can fail and modern solid state ones tend to give less warning before they expire than mechanical ones. There are plenty of other things that can delete or corrupt data so it's best to be prepared.

It may be an old cliché but it's still true: "Data you only have one copy of is data you don't care about."