Why does my phone randomly go on silent? Not vibe silent

[iMore Question]

Every so often it goes on silent by itself. I don't even know how to do put it on silent. Then I try turning it on and off but now it's stuck on silent

 

[SwitchBeach]

What phone?
Define "on silent"? Do you mean .... Do Not Disturb? Volume turned all the way down? Mute switch turned on? Something else?