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

>>17

Im the teacher for the course ;) im designing a course for the school next year and I coded it all from scratch

I aint a dumbass but thats, and I have no where to turn cuz google gave me shit...