Losing all content on Custom CSS sheet

Hello,

I would like to know whether it is possible to restore my custom CSS file. I have put in quite a lot of code into it already in the past week, but out of no reason today (not even a theme upgrade or anything), my website refreshed as i was working on it and I have lost all codes I had on the custom css page (and the page is now like when I first installed the theme, just blank with a few lines saying what the page is for.

I am using the Kalium theme and had used the Custom CSS sheet provided under the theme options (and did not make any alterations to the actual theme css file itself).

Please let me know if there is any way I can restore my work. I would also like to understand why that is happening because my understanding was that if i were working on a custom css sheet, any theme upgrades should not have affected it at all?

Thank you & regards,
Carole