If the fan started and power button lit up, I really doubt that it's the power supply.
There is a screen that comes up from my motherboard at the start of the boot (although it isn't often visible because of the screen booting) and I'm assuming yours would do the same, so if you're not getting that, I think you're right and it is a motherboard failure.
There's a bit of a pain about replacing motherboards, because Windows assumes that that's the core of the system and if it changes, you've tried to install their software on a different computer. Also, you have to unplug and reconnect EVERYTHING.
There is a screen that comes up from my motherboard at the start of the boot (although it isn't often visible because of the screen booting) and I'm assuming yours would do the same, so if you're not getting that, I think you're right and it is a motherboard failure.
There's a bit of a pain about replacing motherboards, because Windows assumes that that's the core of the system and if it changes, you've tried to install their software on a different computer. Also, you have to unplug and reconnect EVERYTHING.