|Then you probably either connected the cables wrong or you dislodged something. Try reseating all cables cards and RAM. |
Of course even just touching the components inside the case can release a static discharge. Especially now that the air is dryer in winter.
If you dislodged the heatsink then you may need to remove it clean both surfaces and apply new thermal compound before refitting the heatsink.
If you are not getting any beeps or video you may have disturbed the RAM sticks. Remove them and re-insert.