OK,
I have come across this problem at least twice: PC doesn't open. You reboot and you see for a coule of seconds the BIOS diagnostics page. After that nothing. You reboot again, nothing.
After a day or so if you try to open it you have the same situation.
Has anyone of you come across this problem?
Is it the motherboard or the power supply?

Thanks in advance

It may not be either. Have you opened the case & looked inside? Checked for dust buildup or possibly a bad CPU cooling fan?

I just finished with a similar problem.

Turned out to be a bad external, powered USB hub.(!!!)

Try disconnecting everything and plugging them back in one at a time

Larry

It usually takes more than three weeks to prepare a good impromptu speech.
- Mark Twain

Some bioses will go to the bios Setup pages and say something along the lines of the bios settings have changed and you should check the settings, if you press the key to get into the bios Setup at the wrong time while booting - in that case it's a false message.

1. Most frequent reason:
Failing power supplies are common and can cause your symptoms.
Check your PS.
See response 4 in this:
http://www.computing.net/hardware/w...

2. This is also a common cause of no boot:
A common thing that can happen with ram that worked fine previously is the ram has developed a poor connection in it's slot(s).
This usually happens a long time after the ram was installed, but it can happen after moving the computer case from one place to another, and I've had even new modules that needed to have their contacts cleaned.

See response 2 in this - try cleaning the contacts on the ram modules, and making sure the modules are properly seated:
http://www.computing.net/hardware/w...

If this is a laptop, you must remove both its main battery and AC adapter before you do that.