This sounds as if a connector or pluggin adapter has become UN connected. A disassembly would allow the checking of these connections but my experience says the GPU (video) card or chip has come unseated or been damaged in transport, thus the same display internal or external.
I recently built a computer and it has been working well for about 4 months. I moved home from school a few days ago, and when I turned my computer on the colors were all wrong and there are horizontal lines all over! I've tried plugging in my monitor to the motherboard's display output instead of my video card's, but the same exact thing is happening. What can I do to fix this? (Also, I took a screenshot, but I can't post it in this thread)