All my characters have become question marks "2"

Dear sir. I’m writing to ask you a favour. I’m having a problem when using your programme “LibreOffice Calc” I made a very important project on your programme. But when I supposed to finished it, there was something wrong. All symbols in tables turned to ? question marks. I’ve spent a lot of time for this project. Unfortunately, I am not able to do it again. Could you help me with this problem? Thank you so much. I have “windows 8.1” Program “LibreOffice Calc” v., language russian and english .

Share a file. Your description contains no useful information that could help identify the problem. For starters, what is the file format?

Learning to ask smart questions will help too.