Chillblast PC dead - Press On and a flash from Power-on LED then nowt

Windows 11
Mobo - P8 777-V Pro
PC around 4 years old.
Disk monitor reported drives healthy apart from one that was on its last legs.

My PC was working hard over past few days copying a lot of data from the failing HD to new SSD. Process had been going well over a number of sessions, but then according to copying app, appeared to lock and copying stopped, but activity LED on On/Off switch continued to light. No response to Kbd or mouse, so couldn't use any of the usual aborts including task manager (no response to Ctrl-Alt-Del). After a lengthy period (~ 10 minutes) had no option but to press On/off switch. That too failed, so waited a bit longer but still wouldn't work and HD activity light continued to be displayed. Only option left was God switch - pulled its main power supply!
left for some hours and then tried to switch on again. Saw a very brief flash from Power on LED on the mobo and then nothing.
Stripped PC down to absolute minimum - still nothing other than the brief power on LED flash.
Checked obvious connectors - all OK and in any case no reason why any should have suddenly disconnected.
I have built PCs in the past so have been happy to get my hands dirty but frankly as an octogenarian I am now too old to feel up to major tinkering.
Unless there is something obvious/simple like a fuse, my options are to Chillblast or take to local PC shop.
Any suggestions would be welcome.

Hi,

It looks like your PSU has blown, Or it could be the MB... Both will need a bit of tinkering...

Go to a Computer shop and ask for a diagnostic...

If it is only the PSU, it could be quickly repaired for a reasonable price,

If it's the MB... Change everything.

I also use a PC from Chillblast (for 8 years) and have found their staff to be most helpful, even years after purchase. I suggest that your first step should be to ring them and talk through your problem. A phone call won’t cost anything, and their technician might give you a good pointer towards a solution.

Thank you for your responses. I expected that they would confirm my own thoughts that it's either the PSU or MB but I hoped, but didn't really expect, that by some miracle someone would come up with a cheap and simple answer - like Samuria's (which sadly didn't work)!
However I will look on the bright side. If it is the MB, it's an excuse to upgrade.
And yes, Chillblast provide excellent excellent support so will give them a call tomorrow.

I would suggest unplugging all peripherals - unplug all the internal disks as well. You just need to pull out the sata power cable that is on the back of each disk. I have known dodgy disks to prevent the machine from even getting into bios.

Then see if you can boot into bios. If you can then that points at one of the things you detached ( or possibly one of the cables/sockets they were attached with).