My PC won't boot

[mac and linux user]

My PC that my dad's friend built 5 years ago will have its lights turn on then seem like it turns off and on again but nothing displays on the screen.

I do not know the full specs but I can give an idea.

Cpu: 5th gen Core i7 or 5th gen Core i5.

Gpu:none.

Hard drive: Sata 3.0 750GB 2.5 inch Seagate 7200rpm 

Mouse:Random Crayola mouse I got from my mother.

Ram DDR3 3200mhz Corsair

400W power supply.

 

[mac and linux user]

Or the ram might be 2400mhz.

[mac and linux user]

I am also going to upgrade the HDD to a 1TB NVME M.2 SSD and maybe put a cheap graphics card in.