My save data disappeared!

[ima]

I just wanted to acquire the demon's mask in Sakyubasu no Tatakai again, but my save data just gone!
And i literally didn't do anything with my browser or anything else, so why would it happen?
Is there any chance to return my progress? (It's like hours of playing)

p.s.
Here is a little FanArt as entartainment for you.
@Gorpete of course.