It is important to backup your data. A backup is a copy of your Zotero library file folder. https://www.zotero.org/support/zotero_data
Step-by-step instructions for the PC. http://libguides.northwestern.edu/c.php?g=114874&p=750370
Note: If you are using a MAC, the zotero folder is located, by default, inside your Firefox profile folder, which (be default) can be found at ~/Library/Application Support/Firefox/Profiles/xxxxxxxx.default/
Warning: Before you copy, delete or move this folder, make sure Firefox and/or Zotero stand alone are closed. Not closing Firefox before copying these files can damage your data.