Hit F6 a few times to make the filmstrip appear and disappear. If that doesn't fix it, quit and restart Lightroom with the option and shift key pressed. Then reset the app's preferences.
Short answer: Try to turn off the setting Window > Show Tab Bar, then close and restart Lightroom.
Quoting an answer from John R Ellis in this thread: Lightroom filmstrip images are too big :
This bug has been acknowledged by Adobe: Filmstrip is covering / overlapping filter- and breadcrumb bar | Photoshop Family Customer Community . Please add your me-too vote and details of your issue to make it more likely Adobe will prioritize a fix.
There are two known workarounds. See this post for the first workaround: Filmstrip is covering / overlapping filter- and breadcrumb bar | Photoshop Family Customer Community . Others have reported that resetting LR's preferences also fixes the problem: http://www.lightroomforums.net/showthread.php?25197-Resetting-the-lightroom-preferences-fi le-updated
I hope this helps! Let us know.
F6 didn't fix but resetting the preferences etc did the job.