Big problem...need help pleses:(

hi...
i built this system bout 2 months ago:
antec skeleton case
element series 850 w silverstone psu
hd 4890 graphics card
x-fi elite sound card
p6t deluxe v2 mobo
core i7 cpu @2.66 Ghz
1333 Mhz ddr3 ram @ 12 GB

all was working fine till 2 weeks ago...monitor displayed ''no video signal check vga cable'' and promptly goes to sleep. sent graphics card to manufacturer and was told in working order. I have been advised that if not the g./card the prob then can only be mobo or cpu.
took out cpu and found that the thermal paste was grayed out ...( is this normal?). kind of sticky/runnyish ( but not majorly runny as cpu socket on mobo seems free of any of the paste)
also the last time the pc was working ...it was switched off by power button...not regular shut down...could this be the prob?

thanx guys for any feed back...