Do you mean that it's as if you've tapped the square button (next the the home one)? Assuming you're not brushing that, it could be a resource issue. Clear recent apps - there should be an option when you tap the square button but, if not, just swipe them away one by one. Now go into Settings|Storage and look for Clear Cache. That clears the system cache and is non-destructive - you won't lose anything. Finally, restart the phone. If that solves the issue, you're done. If the fix is only temporary, it may be necessary to do a litle housekeeping so post back here.
