How to reset emacs to save files in utf-8-unix character encoding?

I have a problem. I found out that emacs recently stopped to save all my new files with the default character set "utf-8-unix". I do not understand what I did, but when I open a file, above the mini-buffer I see "--:---" instead of "-U:---", where the "U" says that the file is saved with utf-8-unix charset. How can I reset emacs to save files in the proper coding system???