Using Time Machine is great — it just doesn't do you much good unless you know how to restore from it, and we're not the ones to help you with that, no matter what medium we use to provide tech support. But it really is as simple as opening the Zotero data directory in Finder, going to "Enter Time Machine" from the Time Machine icon in the menu bar, clicking back through the history of that folder until you find the backup from before the files were deleted, and restoring that zotero.sqlite — which should be significantly larger than 983 KB — to your current Zotero data directory, replacing the empty 983 KB file that's there now.
if I sync and my online library isn't fully up to date, then what happens?
Then you end up with data as of the last time you successfully synced.
Oddly, from the looks of it, even though I see sync attempts going back years, your online library only seems to have data up through October 2018. It's possible you were getting some sort of sync error — indicated by an error icon next to the sync button — that went unaddressed.
And how do I check to see if the online library is fully up to date on this site?
Just click Web Library at the top of this page and sort by Date Added descending.
Thanks for this information. It is clear and useful. I have not seen an error icon next to the sync button on Zotero before. Yes, none of my files that I have uploaded to Zotero since 2018 appear to be on the web library. Which is almost all of the files (thousands more than are there now) that I obtained for my literature search for my dissertation proposal that I am currently working on.
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Oddly, from the looks of it, even though I see sync attempts going back years, your online library only seems to have data up through October 2018. It's possible you were getting some sort of sync error — indicated by an error icon next to the sync button — that went unaddressed. Just click Web Library at the top of this page and sort by Date Added descending.