my 420z not respond,why?

hi all

i have a hp workstation z420 motherboard

i try to boot but it won't post [Do not see anything on the screen]

my system: [About a year ago I bought it on eBay]

motherboard: Workstation Z420

cpu: Intel Xeon E5-2660 2.2GHz

ram memory: M393B5170GB0-CK0Q8 Samsung 4GB PC3-12800 DDR3-1600MHz ECC Registered CL11 240-Pin DIMM

GPU: 780 TGX EVGA [Working]

At first I put two types of memory  in two places on motherboard,
Does not work there was a 5 times red led blink,I have seen on the HP site that 5 times this mean ram memory
so i took out one of the memory  and try to Start up again...

and the red led blink Changed from 5 to 8 times
But still no display responds

Maybe the memory is not fit for the processor I do not know

please help

many thanks to answers