Disapearing code O_O

I keep writing up code files with Java. I'm making a game for a class I'm in, and every time I restart the computer AFTER SAVING, the code file deletes all the text in it. Netbeans says it has invalid characters, so I think it changed all my code to a blankpost character....

T_T how do I get it back i needs it? and what is wrong? how do i fix it?

Thanks

>>7
This is some pretty dated documentation FYI. Still good for a beginner of course, but I'm sure something better than this link might exist. Too lazy to search for this idiot, anyway.