It is likely the Motherboard. Normally when a System fails at that level but still demonstrates it is receiving Power the Motherboard has failed. One thing you can try is unplug the system from the wall and hit the power button to drain the Board of power completely. Make sure there are no self powered USB devices as this will stop the Board from clearing. If you still have no joy its possible you are looking at an RMA.
When you are asking for help, always start off with the system specifications. Then, is it a new build that has never worked or is it a previously working computer that now doesn't? Is it an upgrade that you are having problems with?
The Great Carnack is dead. We need you to tell us these things. Each of the three problems above requires a somewhat different troubleshooting technique.
However, onward to some systematic troubleshooting techniques. If a new build, start here:
Breadboard with just motherboard, CPU & HSF, case speaker, and PSU.
Make sure you plug the CPU power cable in. The system will not boot without it.
I always breadboard a new build. It takes only a few minutes, and you know you are putting good parts in the case once you are finished.
You can turn on the PC by momentarily shorting the two pins that the case power switch goes to. You should hear a series of long, single beeps indicating memory problems. Silence indicates a problem with (in most likely order) the PSU, motherboard, or CPU. Remember, at this time, you do not have a graphics card installed so the load on your PSU will be reduced.
If no beeps: Running fans and drives and motherboard LED's do not necessarily indicate a good PSU. In the absence of a single short beep, they also do not indicate that the system is booting.
At this point, you can sort of check the PSU. Try to borrow a known good PSU of around 550 - 600 watts. That will power just about any system with a single GPU. If you cannot do that, use a DMM to measure the voltages. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire (standby power supply): 5 volts always on. The green wire should also have 5 volts on it. It should go to 0 volts when you press the case power button, then back to 5 volts when you release the case power switch. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.
The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.
A way that might be easier is to use the main power plug. Working from the back of the plug where the wires come out, use a bare paperclip to short between the green wire and one of the neighboring black wires. That will do the same thing with an installed PSU. It is also an easy way to bypass a questionable case power switch.
This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.
If the system beeps:
If it looks like the PSU is good, install a memory stick. Boot. Beep pattern should change to one long and several short beeps indicating a missing graphics card.
Silence, long single beeps, or series of short beeps indicate a problem with the memory. If you get short beeps verify that the memory is in the appropriate motherboard slots.
Insert the video card and connect any necessary PCIe power connectors. Boot. At this point, the system should POST successfully (a single short beep). Notice that you do not need keyboard, mouse, monitor, or drives to successfully POST. At this point, if the system doesn't work, it's either the video card or an inadequate PSU. Or rarely - the motherboard's PCIe interface.
Now start connecting the rest of the devices starting with the monitor, then keyboard and mouse, then the rest of the devices, testing after each step. It's possible that you can pass the POST with a defective video card. The POST routines can only check the video interface. It cannot check the internal parts of the video card.
Another alternative is the that Motherboard has been nuked by a faulty PSU leaving you with two failed components. Sometimes if you check out the capacitors on the Motherboard you can see leakage occurring which is highly indicative.
My reasoning that the Motherboard is the main suspect is the consistency of the symptoms. Often a faulty PSU will deliver a series of random events such as BSOD issues or random shutdowns.
However in the event the Motherboard is found to be faulty the PSU must be eliminated as the cause of the Motherboard failing or the same thing will happen to its replacement.