That can be irritating. You upgrade your browser, or make some other change to your browser or system that causes your contacts to disappear?! It does happen.
The best way to protect against that is to backup your contacts. Go to the contacts list in your email and export them as a .csv file to your desktop. Then move them to your backup drive. This should be done regularly to keep the backup updated.
In AOL, open Contacts, click More and then Export. Follow the prompts. In Gmail, open Contacts, click More and export following the prompts. Similar steps are found in other email providers.