problems rebooting after memory upgrade

upgrading from 8G to 16G. followed video instructions, no problems.

used Timetec 8g DDR3L 1600. suppose to be compatible with stock PC3L 12800.

upon reassembly and rebooting, power on gets the tada sound, however, the apple logo and progress bar are not visible. the screen is dark (like no light). nothing happens.

disassembled and reassembled. same problem tada sound and logo and progress bar do not appear (screen is dark).

I reinstalled my original 8g's to see if I could get that to work. however, it did not. I have the same results with original memory now, TaDa sound, no logo or progress bar.

what should happen after getting the TaDa sound? what can I do?

thanks.

Dan,

how would I attach an external monitor to the imac? I don't see a way I can do that. firewire??

what are my chances of getting another cable from motherboard to display? I hate screwing up!

if I was able to connect a monitor, would I see the normal boot process take place?

thanks

frank