boot PC not always booting

[Protongog]

Hi guys 

Hoping to get a bit of help with a boot issue.

I have put a PC together from a few spare parts and one or two new ones, it's just going to sit in my front room to be used for casual gaming. It was working fine when I built it but left it sitting for a few months.

When i turned it on the screen goes blank and it fails to even show the bios so I unplugged it checked the cables and rebooted all was fine but when I try to reboot it goes blank again. I tested the on-board graphics and the same thing happens. If I fiddle and unplug it boots but a reboot will not work.

Help please. 

[userzero]

Monitor, PSU, Cables, Motherboard. Could be anything.

[Protongog]

The problem has changed now. It goes to the bios shortcut screen (us megatrends) and just sits there.