Weird bios / startup problem

[Kanieos]

Hi everyone, I hope that maybe someone can help me on this:

My grandfather had a rather old pc which was getting really slow. 

So I upgraded the pc with my old spare hardware (CPU, Mainboard + GPU came from my old hardware) plus I bought a new sata ssd and put it in there. 

The only thing I didn't change is the PSU. 

The specs are now:

CPU I5 3330

Mainboard gigabyte h77 ds3h 

GPU gt630

Crucial Mx500 1tb sad

 

I now know, that the PSU is faulty as well, I thought it was his old Mainboard, but it's the psu. The pc only starts when the psu is fully discharged (plugged off for a few seconds), else nothing happens when hitting the power button.

Since his old pc worked with this pc for years (although it had the same problem for years) I told him that he can use the pc normally and I would install a new pc next time I'm around. 

Now there is a new problem: the pc doesn't boot into windows, only the gigabyte start screen is showing. 

When pressing buttons to enter the bios or boot menu nothing happens. 

After a few seconds the pc tries to restart and only shows the gigabyte screen again. 

So what the hell could cause the pc to refuse to enter bios?

At the moment only my mother is at the computer and I'm on the phone giving instructions what to do. 

Tomorrow I will visit my grandparents and look after the pc myself, but if anyone has an idea what to look out for I would appreciate it.