Another possibility.
It's possible that you may have mistakenly used a feature called "window roll-up". If you "roll-up" the window, you will only see the title bar when you maximize the window. If it maximizes over another open window, you may not even notice it -- it may camouflaged by the other window. Or if it maximizes along top of screen, you just might not notice it because you're not expecting to see just a title bar. Here is a screenshot where I put a rolled-up window near middle of page:
Depending on your settings, you can un-roll the window by either clicking the "-" on the left side of that title bar, or by rolling your mouse wheel forward or backward while hovering on the title bar. You can set that behavior in
Settings Manager -> Window Manager Tweaks -> Accessibility (tab).