I’m very sorry to hear that!
All of your projects are stored locally in you browser. If you switch browsers or you clear your computer’s history (including the data stored on websites), you will lose the projects you are working on. Did you download or share your projects at any point? If so, you can import them back into the editor.
We have some new features coming to prevent this. In the meantime, make sure you save your projects locally or share them. When you download a project, you can drag the PNG or UF2 file into the arcade window to import the project.