You (or someone with access to your library) deleted all the items earlier today. This could've happened if you mistakenly use Replace Online Library from an empty local library, which people sometimes do by mistake while trying to set up a new computer. (Setting up a new computer simply involves syncing normally.)
If you were syncing multiple computers, or if this was done on some other computer (e.g., a shared computer where you left yourself logged in), you could likely restore from one of the automatic backups on the computer where you were using Zotero previously and then use Replace Online Library, as explained there.
Upgrade Storage
If you were syncing multiple computers, or if this was done on some other computer (e.g., a shared computer where you left yourself logged in), you could likely restore from one of the automatic backups on the computer where you were using Zotero previously and then use Replace Online Library, as explained there.