PC won't turn on after months

Hi everyone,
I’m trying to sell my old PC, but I haven’t powered it on for about 3–6 months (maybe turned it on once during that time). Today I tried powering it again and it won’t turn on.

Here’s what happens:

• The motherboard RGB lights flash red once.
• When I press the power button, nothing happens.
• I can feel electricity flowing to the case, so power is definitely going in.
• When I switch the PSU power switch off , the motherboard RGB lights (just rgb lights) up normally for about 2 seconds, then turns off again.

I’ve already checked all the cables, reseated everything, and even cleaned and tidied the back-side cable routing, but still nothing. I buy this pc back in 2023, so it isn't that old.

Based on these symptoms, what do you think the issue is?