findersidebar
My finder's sidebar is empty (see attachement), this happens about once every two weeks, I would be working on my mac and suddenly the sidebar becomes empty.
It works again after a restart, but it is a great inconvenience to have to do that each time.
This might be caused by the general slow spotlight problems with OS 10.7.5 and 10.8.2. What version of Mountain Lion are you running? How long has this been a problem? If you're running 10.8.2 you might try this workaround.
This happens when that separator needs to accomodate space for a scroll bar due to content length. You can resize them with the handles at the bottom end since 10.0.
Best Answer
The first thing I would do is run a cleaning utility (like Onyx), reboot and see if cleaning up caches, running disk repair, maintenance scripts, etc. does the trick.
Failing that go to Apple support and download the latest Combo Updater that matches your current OS version and run that. Combo updaters often fix odd macOS behavior.