Failure to POST, MAG Z790 Tomahawk WiFi

A recent build that was working fine, but now isn't...
CPU i7 13700K
SSD Western Digital BlackSN850X
Ram Trident Z5 16G x2
Water cooled
Geoforce RTX 4070ti
My daughter's puter, about 6mths old. The puter has been switched off for about 8 weeks. Recent start up, get amber led on for about 3 seconds, and then off, followed by the red-cpu led on briefly and then shut down. Have removed ram, and cleaned contacts(soft eraser) removed graphix, removed ssd, hell, I even removed the cpu.
So it's either MB or CPU...
Thoughts?

Did you make sure all the necessary power cables are plugged in correctly and didn't come lose in those 8 weeks? Especially the CPU Power cables and ATX 24 Pin you should double check.

Thanks for your reply. Yes, I have pulled all motherboard cables and reseated them. I am going to remove the power supply next and see if it will power up a known 'worker'. It's all a bit odd.