And the file is my thesis, witch is almost done. And I`m desperate! What can I do? Texmaker opens my other (unimportant!) files fine, but this one seems weird, it sort of slows down the whole Texmaker and it looks like the file is empty and I don`t know... It was fine earlyer today. What did I do? Or what happened I really don`t want to write it again. And feel so stupid that I haven`t made backup in a while...
At first, make a backup now. Copy all of the files of your thesis folder to a USB thumb drive or somewhere. Even if the .tex file would be damaged, you may keep your pdf output file for re-writing if necessary.
Try opening your .tex file with a simple text editor, such as notepad. But before you try anything, make a copy of all.
Then - Texmaker is just an editor. If it has problems, you can use another editor. For example, you could install TeXstudio: it is similar, it bases on an earlier version of Texmaker is was further developed. I use TeXworks as editor.