First I'll say that "Restore from Zotero server" should not be used under normal circumstances, especially without fully understanding what it does. There is a warning advising against its use on the same preference page.

"Restore from Zotero server" does what it says, which is copy data from Zotero server to your local database, overwriting anything you have locally. Unless you also used "Restore to Zotero server", there would be no way to lose data on the server. If you had synced previously, and you're sure that the data was online, then it's likely still there and you may have used the wrong account to sync.

Could you be more specific about what you mean by "online" though? Was it on zotero.org or was it a library you had in Firefox?

Do you have a backup of your hard drive? Did you make a manual backup of Zotero database? Are you using Zotero on another computer?

Please make a manual backup of your current Zotero database before proceeding with any further steps https://www.zotero.org/support/zotero_data#backing_up_your_zotero_library