Without looking up the specs for your kit in order to check compatibility:-
If you have on-board graphics, boot without the card and with 1 module of RAM. If it doesn't boot, try each module of RAM at each reboot attempt. Make certain you are in the correct slot for 1 module of RAM.
With all builds, take some time to check all your connections by removing and re-reconnecting.
Don't lose heart - you will have pleasure sorting the problem which I suspect is something simple ;-)