Think about what you may have done on your computer recently, added a program, updated, or upgraded something. Thunderbird may have been corrupted. Next time you can export your address book. Go to tools and click on export. Save the file with a name you will remember. The file extension is LDIF. You may not need it but a backup is always a good idea. Take some time and send in more details. Check your editing and spelling. Sorry to be the one to say, but your question will get more response if it doesn't have typos.