ZoteroBib stores entries in the local storage of your web browser. If you switch to a different computer, use a different browser, or clear/reset the history of your browser, you'll lose your ZoteroBib entries. If you have multiple browsers on your computer, try other ones you might have used.
But ZoteroBib is really intended for one-off bibliographies. For longer-term storage, you should use Zotero instead. If you're not going to use ZoteroBib for a while or want to switch computers/browsers, you can also save a copy of your ZoteroBib bibliography using the "Link to this version" option, which gives you a URL you can use to retrieve it.
Upgrade Storage
But ZoteroBib is really intended for one-off bibliographies. For longer-term storage, you should use Zotero instead. If you're not going to use ZoteroBib for a while or want to switch computers/browsers, you can also save a copy of your ZoteroBib bibliography using the "Link to this version" option, which gives you a URL you can use to retrieve it.