Thunderbird (an email desktop client) has a similar issue: some keystroke (I don't know what, but I've accidentally hit it a few times) collapses the folder view on the left. The only way I've found to restore it, is to mouse-drag the dividing line back right. But it's not clear there is a dividing line, you have to guess that it's there, and be excruciatingly careful about positioning the mouse cursor.