Oh sorry, forgot that, then check RAM sticks, like I said only let one stay in there, put it into the second slot from the CPU. If it isn't working, try the other one. I also hope you have the RAM sticks in the right slots, you want them in the second and fourth memory slots, they have the same color.
A quick question: you cleaned your CPU recently, applied new thermal paste, and now it isn't working, or you did that a year ago(like in the title) and now all of a sudden it isn't working?