"You won't believe it! I was in the bathroom at a restaurant and a button fell off my jeans! So embarrassing."
I generally use 'bathroom' to mean anyplace with a toilet - restroom being more formal. Saying "fell off of my jeans" sounds more proper to me but in a non-formal setting I'd omit it.
If you still have the original button that popped off you'd say that you're reattaching it - "I need to have the button reattached." If you need a new button, you'd want to have it attached - "I need to have a new button attached."