Dont't boot anymore since my power supply died

Hello,

I have a ASUS P5GD1 motherboard with a Intel P4 3.0. It was feed by a 300W power supply and it died (no boot, no led and smell burn...)

Then I bought a new ATX psu 450W, I connect the 24pin and 4pin connectors then now power is back (green led on the MB) but the system doesn't boot. It does 1 beep and stop at "Asus P5GD1" screen. I tried without success to enter the BIOS with the DEL key. If I press the DEL key right away when I power on the system, it stop at the "American Megatrends" screen and the last line is "Initializing USB controllers...."

I tried to replaced my new PSU for a 550W but got the same issue. Any idea ?

Thanks for your help.

Jean-Francois