my two restoration PCs are dead!!!

[spoRv]

They remained switched off for few months, and now they turn on, but nothing more... hope to find a way to fix them, or make one working out of two, or, at least, that the HDDs are fine...

I'm very worried about the data... more than three years of work...

If the computers simply don't boot, it's not the HDDs that are at fault. You can boot a PC without a HDD at all.

[spoRv]

I know, but until I have them tested, I could not be 100% sure...

Sounds like a PSU problem. Or pulling the motherboard battery and resetting it might work. I have one decade old PC that sometimes needs doing that. But if there's just a light and it doesn't do or show anything, it's probably the power supply.

I was thinking testing the coin battery also.

[spoRv]

Thanks, I will try everything in the next days, and report results here.

[Evit]

Try replacing the battery first. If not, reset the motherboard with the jump key or whatever is called