Sorry to hear you had this problem. I expect that your work is not actually lost, but only that the Dorico folder doesn’t appear in your iCloud Drive, so you can’t see the project in the Files app.
You should find that your file can be found by looking inside the hidden folder via the Terminal app on your Mac. Run Terminal via Spotlight, then at the command line paste the following command:
cd ~/Library/Mobile\ Documents/iCloud~com~steinberg~iosdorico/
and hit Return. Then type:
ls
and hit Return. You should see the name of your project there. Let’s assume your project is called My Project.dorico. You can now type:
cp "My Project.dorico" ~/Desktop
and hit Return. This will copy the project file My Project.dorico to the desktop on your Mac.