The system shuts down shortly after booting

Hello,
After starting the PC it enters the booting process but shuts off within 5-6 seconds. LED lights and lights on two IDE drives come up, the fans keep running but I get a message on the monitor saying "No signal detected". The reset button does not work. I have to shuoff the switch on the power supply to shut the PC. Even if I enter del to work on BIOS the same thing happens within 5-6 seconds