For some reason my save was deleted after I lost one battle and wanted to reload
Viewing post in Mahō Shōnen: Magical Battler comments
The saved data is stored in the browser. This means that playing on incognito mode won't let you save, and that having setting that erases tracking data or other information (a common procedure for security) might be also cleaning up your saves.
If you are confident your situation is not any of the above let me know which browser you are using and I'll see if I can reproduce the issue and figure out why this happens.