I lost all my workspace

I don't know how to fix that, honestly I think your workspace might just be gone.
However, what I do is I save multiple copies, so that if something goes wrong, I can simply load up one of those copies.
I think that in this situation, you'll just have to start over. I had to start over on my post-apocalypse mod due to one update, then 1.13 came out, so now I'm waiting for an MCreator version compatable with 1.13.

MCreator saves backups in the workspace directory in the form of .backup-0 and .backup-1 files (just remove that in the name and have the extension as .mcr and you can then import it). You can find them in the ./user/workspace directory (with the . being the mcreator directory)

Every time you open MCreator, it saves the current workspaces as backup-0, but the backup-1 should be safe. You can see if your mods are in it by the relative file size. About 5kb is probably an empty workspace.

But as stated above, you should export copies of the workspace as your own backups, which you can import in case something happens to the MCreator backups.