The cloud storage is implemented in using git as backend. That is a version control system that includes every operation performed on your log, and it is possible to go back in time. However, Subsurface does not have user interface for this. Both the mobile and the desktop versions of Subsurface have a local cache of this repository, and in desktop version it is also possible to export the dive log in XML format, if you want to have an extra backup of the log. If you are not familiar with git, you should send an email to "dirk (at)
hohndel.org" from the email address you have associated with your log, and give him explicit permission to access your log on the cloud server, explaining the situation and telling what state you wish to end up to.
However, if you are familiar with git, you can do the magic yourself locally and push the fix to the cloud. I just don't know what path the cloud log is stored locally. If you start Subsurface from command line with -v option, it should print out the local path. You might need to checkout a branch named as your email address, and then you can e.g. revert the commit that overwrote the log with empty data.