Disadvantage of using differents rams ?

hi,

After buying new ram for my pc (corsair vegeance 2x4gb 1866mhz) my pc won't turn on. All fans are spinning, pc makes usuall sounds, but no video output, keyboard and mouse doesnt light up. Tried reseting cmos by removing the battery. Swiching back to old ram same problem. All cables are checked and connected properly.

if i use 4gb of ram and other of 8gb wich are the Disadvantage like the dual mix?

I didn't find the right solution from the internet.
References:
http: // www.tomsh ardware.com/answers/

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