There is also available in most (not all) MyBinder-backed sessions an opt-in option to save noteboks along the way to your local browser cache. Had you done that along the way, you may have been able to restore from there? Maybe??? I’m unsure as I try not to rely on it. And it is best tested well in advance of a crisis.
See Getting your notebook after your Binder has stopped.
In case it looks like it isn’t included by default in the older notes about it (or the notes about it don’t show JupyterLab examples), I’ll clarify. Originally, you had to add the extension in to the configuration files for your repo; however, now this ‘safety net’ system is included by default in typical MyBinder.org-served sessions without the need to specify anything in your configuration files. (Those backed by Dockerfiles, which are not recommended, don’t include this though at this time.) It is available in both the classic notebook interface and in JupyterLab in launches from typical MyBinder-ready repos.