My acer aspire 5517 will not boot up

Hello,

my acer aspire 5517 laptop will not boot up but the power light comes on and the fan and hard drive and cd drive start as well the screen flickers when you first press the power button I recently did a hardware diagnostic and it told me im only runnin at 256megs of ram but I have a 1 gig ram card and a 2 gig ram card so all in all im runnin 3 gigs why would it tell me im only runnin at 256 megs and when I took them out to look at them and then put them back in the same way my laptop started actin up by not powering on please help me