Is there a Zotero Quick Start Guide in the library you see? If so, what's its Date Added in the right-hand pane?

Do you see a 'zotero' directory in your Firefox profile directory or your Zotero profile directory with more files in 'storage'?

In earlier versions of 5.0 if there multiple older data directories it didn't always choose the best one, but that shouldn't be the case anymore, so we'd want to figure out what happened in your case.

What files do you see in the root of C:\Users\..\Zotero, and what are their modification times?

You can close Zotero, rename that Zotero folder to Zotero-Old, move the 'zotero' folder from your Firefox profile to C:\Users\..\Zotero, and restart Zotero, and you should see your data again. (And once that's done you can delete Zotero-Old.)

Thank you for your help

I have a similar issue...suddenly I have two Zotero folders in my HOME folder...when I open standalone Zotero I get no files and a warning about syncing into an empty folder...not sure what to do...

Also, is the Firefox version of Zotero gone? Most recent update left me with just a button to save something to Zotero...

Running Ubuntu 14.04...Zotero 5.0.15

I have two folders, one is named Zotero and the other zotero in my HOME folder. I did not have two as of yesterday.

I don't even see a library anymore in Firefox...all I see on my toolbar now is a button to save items to Zotero...I have to right click it to see options. There is no way to view my files as there was previously.

The zotero folder is in my Firefox profile, but how I know if it is the correct one? I do have a backup from a few days ago thankfully, but I don't want to write over my files.

But a "zotero" (lowercase) folder isn't something that Zotero 5.0 would create in your home directory, only "Zotero". Zotero 4.0 used a "zotero" directory within the Firefox profile directory. If you created a "zotero" directory previously and used that as a custom data directory, that might explain that.

In any case, look in the 'storage' folders and you should be able to figure out which of the folders has your data.

The 'storage' folder would be within the Zotero folder. In an empty library, there wouldn't be one, so look in the others.

Oh, that's just your Zotero application directory — i.e., the extracted Zotero tarball, either the one you're currently running or an outdated one you can delete. The install location on Linux is up to you or a third-party package that you're using.

You're looking for the previous Zotero data directory. If you were using Zotero for Firefox previously, it should be in your Firefox profile directory.

That's in your Firefox profile directory? That looks like an empty database — assuming the zotero.sqlite is around 1 MB, there's no data there. What's the modification date of zotero.sqlite?

The bug here was just not properly migrating your old directory to ~/Zotero, so your data still exists somewhere. You can also check the Zotero profile directory for a 'zotero' folder.

If you have a backup of your Firefox profile directory, you can open the prefs.js file and see if there's a line that says "dataDir", which would tell you for sure if you were using a custom data directory.

According to the dataDIR file I was using the default directory.

Well, what do you mean by default directory? The default directory was within the Firefox profile directory in 4.0. In 5.0 it's now ~/Zotero. My understanding was that you just upgraded to 4.0.

OK, well you should do a complete search of your hard drive (including hidden folders) for other zotero.sqlite files. Again, the bug here was really just failing to properly migrate either the old data directory or the old custom location setting, so it should still exist somewhere. It still seems possible you were using a custom data directory without remembering that. You're looking for a zotero.sqlite > 1 MB and a 'storage' directory with additional files in it.