It depends, really. Do you have a black screen and/or won't turn on at all? Or does it beep when your Mac is on? If it just won't turn on and it beeps, it is more than likely it's a RAM issue. Sometimes, it can just as simple as a loose connection, or you can try doing what's called a PRAM reset. Right when you turn on your Mac, press Command+Option+P+R and see if it makes any difference. When you do this, you should hear three beeps, which is completely normal. If it does not make a difference, try removing one of the pieces, I suggest removing them one-by-one to see which one could be causing the problem. Sometimes you can't tell just by physically looking at them, you may have to play around little bit to see which RAM piece is causing the issue. Hope this helps!