Missing Home Button and Recent apps button

[Android Central Question]

after restarting my device my home and recent apps button is gone, the only thing left is the back button. the notification tray still work (like battery, signal strength, clock, etc.) and i still can receive LINE phone calls and Instagram direct message. But i can't open the apps and my wallpaper changed to all black. I've tried safemode reboot but it doesn't work. device : sony xperia m5 android lollipop