Sounds like the display adapter/graphics card went out on the motherboard of the laptop. Either that, or the CPU, Ram, or possibly, the hard drive went bad. Since you did some stuff that would mainly affect the hard drive, I'd probabably trouble-shoot in this order:
1. Take out the hard drive, see if the system gets farther without it. If it does, it's the hard drive.
2. IF you can get the system to run longer than so far, try connecting to an external display/monitor, and see if you can see anything there. If not, then it's definitely something on the board - graphics, cpu, ram..
3. I'd next try removing one stick of memory at a time and seeing if it boots.
I will admit that the symptoms to me sound more like a heat issue or the graphics card going bad. But based on the scenerio, I'd check the above first... not to mention those are the easiest things to check.
You could also check to be sure the air flow isn't blocked.
Also, if you ran the system for an extended period of time, what sort of ambient room temps did you run the system in? Laptops generally run hot, and if you're running in a hot environment, it's a no win situation for sure.