|If you have not been fiddling with anything inside the case, there are two common problems that can cause your problem.|
Failing power supplies are common and can cause your symptoms.
Check your PS.
They often partially work, fans and hard drives may spin, leds may come on, yet you get no video and the mboard will not boot all the way.
See response 4 in this:
A common thing that can happen with ram, even ram that worked fine previously, is the ram has, or has developed, a poor connection in it's slot(s).
See response 2 in this - try cleaning the contacts on the ram modules, and making sure the modules are properly seated: