Zotero 4.0 and earlier stored data by default in a 'zotero' directory within either the Zotero profile directory or the Firefox profile directory.

When you first start Zotero 5.0, it attempts to migrate your data directory to a new default location, a “Zotero” directory in your home directory. (See Zotero Data for the default location on your platform.) If you previously used both Zotero for Firefox and Zotero Standalone and chose not to share a data directory, you may have a Zotero database in both profile directories, and Zotero will automatically use whichever database was modified more recently.

However, if security software on your system prevents Zotero 5.0 from accessing the Firefox profile directory, it will automatically either migrate the data directory within your Zotero profile (if one exists) or create a new data directory, potentially leaving you with an outdated or empty database.

If Zotero 5.0 then syncs with your online library, it will update that database, but if your online library is out of date — for example, if you haven't synced in several months — you could still end up with missing local data.

If you're able to temporarily disable security software or whitelist Zotero, doing so may allow Zotero to automatically detect your previous data directory at startup and offer to migrate the directory for you.

Otherwise, to restore your data, first identify your current Zotero data directory from the Advanced → Files and Folders pane of the Zotero preferences. (Again, this will generally be “Zotero” within your home directory, but it could also be 'zotero' within your Zotero profile directory.)

Then find your Firefox profile directory and look for a 'zotero' subdirectory. If you find one, check the timestamp of the zotero.sqlite file — which should correspond to the last time you had Zotero for Firefox installed — and confirm that there's a 'storage' directory with other directories below it. (These are your attachment files.)

To restore the data directory you located, close Zotero, rename your current data directory in your home directory from “Zotero” to “Zotero-Old”, and then move the 'zotero' directory from your Firefox profile to your home directory, renaming it to “Zotero” (with a capital “Z”). When you then start up Zotero 5.0 again, you should see your previous data.

If there isn't a 'zotero' directory within your Firefox profile directory, it's also possible you were previously using a custom data directory location, which Zotero 5.0 also wouldn't be able to locate automatically if it was blocked from accessing the Firefox preferences file. If you know the location, you can simply point Zotero to it from the Advanced → Files and Folders pane of the Zotero preferences. Otherwise, open the prefs.js file in your Firefox profile in a text editor and search for dataDir to see if a custom path was set.

If you're still having trouble locating your previous data, post to the Zotero Forums with the following information:

• Your current data directory location from the Advanced → Files and Folders pane of the Zotero preferences
• The timestamp and sizes of zotero.sqlite* files within that directory
• Which version of Zotero you were using previously
• The names of a few files you see in your Firefox profile directory