GTX 260 not powering up in 2 way SLI

i suspect you PSU might be the problem. what is your PSU? try removing one card and see if your pc will be able to boot properly

my PSU is the Corsair 850. running on the evga 780i mobo with 8gig XMS ram and four 500GB hardrives. My cpu is the Q9400.
The thing my PSU is still running everything else including the two pumps for my two liquid cooling loops. but my GTX 260 just wont run.

I second the pulling of the newer card and see if it works.

What's the power connection to the 2 cards?

I have two six pin PSU cables going to each GPU. my PSU has four six pins in total.

so I just got back from the campus and did as you sujested but still not even the old card was running. So I tried my old 9600GT and it ran but no signal was been sent to my monitor. My 9600GT still runs by the way.
So I thought the mobo might have something to do with it and I searached for my old 730i and hooked it up and both the GTX 260s appeared to be running just fine.
so do you think something might be wrong with my 780i mobo or what?

if your card running fine on another mb then your mb might be the problem. maybe your pci-e lane can't give enough power to your 260

Your not telling us what else you did in your case today. You said remodeling ? An added one new card. Did you have your cpu out, or your motherboard out ? If not, maybe the board had some kind of failure when you first started with 2 cards? Why ? some kind of load that was not there before ? Sometimes its helps whether its cars, or computers you change only one thing at a time. Good luck.

Ok. in that case, this is exactly what i did.
old setup:
OCZ 600W PSU
8GB XMS ram (still same)
780i mobo(still same)
3 Carvier 320GB Hardrives
Coolermaster 590 case
Q9400 CPU(still same)
one GTX 260

new setup:
Coolermaster 850W PSU
same ram
same mobo
added 1 more 500GB Carvier
Antec 1200 case
same CPU
added one more GTX 260

These are the basic important things i changed( besides all the numerous bay devices and stuff).
but anyways, i did some extensive research today after trying everything for the whole day and found out that am just one of the thousands who have already faced this type of problem with the 780i mobo either after windows installation like me or just after some other times.
Everyone says the mobo has a problem from manufacturer's making and is probably fried up or BIOS chip is now trash.
I tried my GPUs on my old 730i and confirmed they are just ok.
So basically,,,,ma screwed and need new mobo!!!!!!! and am broke!!!!!!!!!! 

Well if the 850w PSU works with the other mobo as well, there isn't really anything else that I think could cause that than the mobo