Sounds like it could be that a component has become loose inside. If the graphics card is loose and can't be detected, then your system won't boot. I'd expect to hear some kind of beep code to tell you that but it wouldn't be the first time that the BIOS has failed in its duty to provide a useful beep code to tell us what's wrong.
My recommendation would be to open up the case and inspect the components. Look at the graphics card and any other cards fitted into the PCI slots. Are they all sitting squarely and snugly in the slots or are they poking out more at one end? If necessary, remove the card(s) from their slots and then push them back in firmly along the full length of the copper connection. Then remove the memory modules and re-fit them, again firmly along the full length of the connection - make sure they 'click' into the slots and the retaining clips at the ends are locked in. If all of that looked OK, you'll need to remove the processor and fan and refit those as well - be careful with the processor as bending one of the pins on the underside can render it completely useless. You might actually want to test the system again before you go through removing the processor / fan (which can be a little awkward) just in case it is already fixed.
Let us know how you get on
