Smoke came out of my iMac, what to do?

My iMac hasn’t been used much so I take all the cables out and just leave it on my desk.

A couple of days ago I tried to turn it on, but suddenly a small cloud of smoke came out of the back, so I turned it off straight away but now it doesn’t turn back on again.

What might be the problem?
Is there anything I need to replace?
I did some research and it looks like it’s probably the power supply, but how do I know if the part I need to replace is actually the power supply and not something else?

I’m afraid to order a new power supply just to find out later that it was something else that needed replacement.

Thanks!