Help with memory

I have a A8N-Sli Asus mobo, and 3 of my ram slots seem to be not working.
I took all my ram out (2 sticks of 512) when i installed my new heatsink on the mobo, and now when i put my ram in any slot except slot3, the computer just does the beeping sound.

The computer only starts up when i put 1 stick in to slot 3 (out of 4), and i tried every other slot and it just does a very long beep on startup...
Do you think my ram slots could have got busted ??

got it sorted now, somehow it fixed itself :-s