System powers up but no boot / "no signal" message on monitor

[hansklossuk]

Since yesterday I have been having this problem. Computer powers up, fans and lights are working so are USB ports and DVD drives. Problem seems to be hard drive - light goes on for moment but seem to go off yet I can feel its spinning. Or is it? Get "no signal" message on the monitor yet everything seem to be working. Since yesterday :

Tried second monitor , spare graphic card and change all the cables
Change battery on the motherboard
Removed all RAM sticks tried to boot with every individual stick
Tried all tricks from YT like keeping finger on power button for 60 sec etc
same problem with new power supply

System is pretty old : P6T Deluxe , GTX650 11GB RAM but until yesterday everything was working

 

[rocktalkrock]

Have you tried booting without the HDD connected? See if it goes into the BIOS.