Alright.
There have been possible solutions posted in the forums, you may check them
here and
here. There was a more recent solution posted, but I couldn't find it at the moment.
Let us know if any of the methods in these links work.
I think, however, that I must mention that this problem has also been reported for other devices - I have experienced this with the L535, but since the L535 doesn't have hardware navigation buttons, I simply resorted to turning the screen off and then on, restarting the phone or performing a soft reset.