Meaning the option in your storage setting on the website?
Assuming you still have your files locally, go to the Sync → Reset pane of the Zotero preferences and use "Reset File Sync History" to force it to check all local files against the online library. Don't use the other reset options.
Upgrade Storage
Assuming you still have your files locally, go to the Sync → Reset pane of the Zotero preferences and use "Reset File Sync History" to force it to check all local files against the online library. Don't use the other reset options.