All entires have gone
I used Zotero for the first time a couple of weeks ago, adding journal articles directly from Firefox. I have just opened it again now and there are no entries in the application, it just says welcome to Zotero and offers me a quick tour. How can I get my articles back? Thanks!
Generally, see Locating Missing Zotero Data.
If not, either something deleted your old data directory (is this a personal machine?) or it's somewhere else on your computer and Zotero somehow ended up pointing at a different folder.
Does Zotero delete entries automatically after a while? I thought that once you saved articles to it, they would just stay there!
I can't tell you what happened, but somehow the Zotero data directory got moved or deleted outside of Zotero, or you're misremembering something about your previous usage. (Is it possible you were using a different account and saving from the Zotero Connector straight to the online library rather than to the Zotero app?) If the former, the linked page explains how you would search your computer for other versions of the data directory. But this isn't something that Zotero itself could have done. If this were the data directory you were using previously, there would be other, older files in it.
Is your data directory set to the standard location, at /Users/:username/Zotero, or is it somewhere else?