OK, we're really going to need more info.
Is your monitor on/connected? Sorry silly question but something stupid like this happened to my mom, she connected her monitor during a move to the onboard (integrated) gfx card, and thought the computer was broken because mothing was displaying on the screen.
Is this a new build?
If not, was it ever booting properly and working? Did you change anything?
Assuming this machine just stopped working all of a sudden it could be a variety of things ranging from the power supply, mobo, or even just the graphics card being fried.
Good luck!