You can not get back the contents. There is no mercy.

But search for backup files. If you are lucky enough there will be a file named hello.txt~ for hello.txt in the same directory. Those files are usually created by text editors like gedit or emacs when you edit a file.

If you have such backup file you can get the contents back (you may get back only a part also). Use the following in the terminal,

mv hello.txt~ hello.txt

It is better to have even a part than losing it completely.