No sound apart from a constant buzzing....... Technical Assessment:
It's buggered 
(Sorry, just teasing; couldn't resist
)
On a more serious note, this isn't something that normally occurs with an incorrect setting. Yes, it's possible to set up an AV receiver with no centre speaker; it's called Phantom Mode. If you did that, then two things: First, the channel would be silent because the amplification to that channel would be switched off. Second, you'd probably remember doing that in the set-up. You can always check this by going back in to the On-screen Display and looking at the Speaker Settings sub menus.
The fact that you have a constant buzzing suggests that something is broken. My guess is that the output transistor pair for that channel has died.
Your next question will be '
Can it be repaired? ' to which the answer would be Yes. The thing is though by the time transport and labour are factored in, plus some diagnosis to make sure nothing else has let go, then you'll be tipping in way more than the cost of a used replacement. In other words, it's unlikely to be economically viable.
Although not your personal experience right now, on the whole, Pioneer AV amps along with Denon, Yamaha, Marantz and Sony tend to be pretty reliable. Onkyo and Arcam are more flaky. Although your current Pioneer is an entry-level 5.1 model, it's pretty well specified. Along with DTS-MA and Dolby True HD, you also have 4K pass-thru, ARC and Standby Thru with the option of selecting a fixed input or Last Used input. Something from around the same manufacture date but a couple of steps up in model will get you more power, better build, greater precision in set-up, a few more inputs but the main thing is the sonics. Something along the lines of a Yamaha 7 series or Denon AVR-X2- or -X3- series, or even one of the higher-end Pioneers could be a nice step up either now or when you're ready for your next speaker upgrade.
I've had two different models go on me. A Pioneer one, too.