If we have to tell someone that their belt is unfastened (a person did that before but now it's come undone)
Your belt's open/untied/undone.
What would sound natural?
No my belt is tied/closed/fastened. (If a person doesn't realize it and feels that his/her belt isn't untied)
The belt can be either be a buckle one or the one that has a hook and is elastic (what girls wear with skirts).