@matthewyim90: Also, though, while your data wasn't in any danger here, and we can always help with problems, there shouldn't ever really be a reason to panic. Just 1) make sure you're using Zotero syncing and 2) make sure you have a backup of your Zotero data directory, and there's essentially no way you could ever lose all your data. (For the backup, you would ideally just use a tool that makes a regular, automated backup of your entire disk to an external drive, rather than worrying about Zotero data specifically.)

it synced everything to an old date

No, syncing doesn't work that way. It doesn't just pick one side and overwrite the other side with that version of the library. Syncing syncs the changes you made on each side since the last time you synced. For deletions, that only includes explicit deletions that you made. If you deleted items on one side or another, Zotero will apply those deletions to the other side when you sync.

It looks like an hour and a half ago you deleted about 1500 parent and child items, and a few hours before that you deleted about a dozen collections. (The former could've been emptying the trash after deleting the collections.) I don't know what exactly you did or why — that certainly wasn't part of any troubleshooting I told you to do — but syncing would've applied those deletions to the other side.

Do you have a backup of your Zotero data directory from before you made these changes?

If not, you can try restoring from one of the last automatic backup, but that won't include any attachment files you deleted.

Not without a backup. As I say, you can likely restore your data from one of the last automatic backups, but that won't include attachment items.

Needless to say, this is why we so strongly recommend following our instructions carefully and not trying drastic things on your own. Nothing was wrong with your data, and there was no reason to delete your entire library.

I thihnk I see the file which is zotero.sqlite.1.bak which has the timestamp of 6/16 10:16 PM which is yesterday. There is a newer BAK file from today at 11:19 as well.

Lastly, there is another SQLITE file that is timestamped for recently 6/17/2020 11:52 which I am guessing is the current one.

Please guide me to undo this and I will not do anything unecessary again. I am so sorry!

I linked to instructions above for restoring from the automatic backup.

You'll have to say more. The instructions are quite precise.

The instructions say to replace the (zotero.sqlite.bak) with (zotero.sqlite.1.bak)

That's not at all what the instructions say. If there's something you don't understand, please quote the exact phrase and say what you don't understand about it.

1) That's not the section I linked to. I linked to the last automatic backup section.

2) While not the instructions you should be following anyway, what you pasted does not say to replace "zotero.sqlite.bak" with "zotero.sqlite.1.bak" as you claimed.

That's explained in the linked section.

Yes, and the section I linked to explains exactly how to replace the online library with the local version.

It's really not about knowing how to use a reference manager or being savvy with technology. We just need you to take the time to carefully read the documentation we provide, and ask specific questions with quotes if you don't understand something. We spend a lot of time making sure the documentation is precise and up to date, and it doesn't help anyone for us to try to repeat it every time someone has the same question.

I am sorry but I cannot seem to locate which link.

It's just the exact same link that I gave twice already, that you've been following, and that we've been discussing for the last 18 posts. From that section:

5. If you were using syncing and the undesired changes were already synced, syncing now would cause the more recent data in the online library to replace the data you just restored.

[…]

III. If many items were affected or collections were deleted, you can use Replace Online Library to force Zotero to upload the local version of the library, overwriting previously synced changes.

OK, glad to hear it's working. Just to reiterate, if you previously deleted many items and then restored from the last automatic backup, it's likely that most of the files associated with your attachments are now missing, and unfortunately there wouldn't be a way to restore those without a proper full backup of your Zotero data directory.

You can add the Attachments column to the middle pane to see which files are missing, as indicated by an empty (instead of filled) blue circle. If you don't have a backup, you'd have to redownload those and either drag them to the parent item (and then delete the broken attachment) or use the Locate option in the file-not-found dialog for the broken attachments to select the downloaded PDFs, which will copy the files back into your data directory (and then you'd want to delete the downloaded files).

OK, even better.