The last time that happened to me it was the video card that was not working. An old spare one fitted saw it all working again. However if the mouse and the keyboard aere not lighting up it is probably somethinf else.
I presume that when you checked tha cables you checked the internal and the external ones. especially the power plugs.
If you have more than one ram module take them out and try them one at a time or beg or borrow a ram module and try that. If it is not the RAM I would think something wrong with the power supply. You could check the voltages with a multimeter.
I have 2 RAM modules I tried starting my pc with one module inserted (tried both) and with none inserted. Nothing happened.
My videocard does get power and its fan is working also. I dont think it's that.
I dont have a multimeter sadly.
But I think my motherboard is broken, I remembered that not too long ago my appartement had some serious power spikes running trough my net. This caused the modem to overheat and kill itself. Maybe my motherboard got suckerpunched too and just gave up now.
Maybe some spikes I didnt notice happened when it shut down but were the straw that broke the camel's back.
I have no idea...