Precision T7400 and diagnostic lights 3 and 4

Hello

my workstation was working fine, but does not boot anymore.

All i get is a black screen, no bios, no post, and only the diagnostic lights 3 and 4.

I looked in the manual, but this light combination isn't mentioned anywhere.

I tried to remove and put back in everything (cpu, ram, all pover cables, vga)

but nothing changes.

Only removing the ram gives me light no 1 and some beeps (memory not recognized)

I also bought some replacement memory directly from Dell (FB-DIMMS DDR2 667 ECC), but same issue.

What shoud i do?

Any help appreciated

Thanks