The online release of Twine 2 stores your project data in your browser's local storage—specifically, Web Storage. Anything which clears your browser's local storage, will delete your project data. The only way to be sure you do not suffer data loss in the online release is to routinely use its archival feature—the Archive item in the sidebar.
That said, it may simply be a protocol issue. You can access the online release of Twine 2 via both the HTTP and HTTPS protocols. Each receives a separate local storage instance from the browser, so it's important that you use the exact same URL when accessing the online release, elsewise Twine 2 won't be able to see your data.
Here are relevant URLs. Try both to see which may have your data:
If you're simply getting a blank page with one of those URLs, try forcing a reload—commonly bound to F5, among other things, in most desktop browsers today.
PS: If you've ever used Publish to create a compiled version of your project and still have one of those files around, then, all else failing, you may import that to recover, at least, some of your project. Though I'd save that as an absolute last resort.