Select Unfold in the Fold menu to unfold a section previously folded with Fold|Fold. The Unfold command will only become enabled when you've placed the text cursor on the first (still visible) line of a folded section. The unfolded section will remain marked as a folding point, so you can easily re-fold it.
If you've made a selection, then all folding points inside the selection will be unfolded.
If the current file type uses automatic folding points, then unfolded ranges will automatically disappear when you edit the file. They will be replaced by new automatic folding points. Folded ranges will remain folded until you unfold them.