If when you turn the computer on, the case fans don't turn on and you cannot eject the DVD drive (and the DVD is connected to power), then the power supply is bad. Make sure it is seated properly into the motherboard (push it down hard). Make sure the switch on the back of your computer is on (switched to "1"). Try swapping out the power supply first. You might be able to spot a kaput PSU by looking for burnt pins or testing it with a multimeter. If you have a multimeter, you do this by connecting the power supply to a power outlet and turn on. Set the multimeter to test for DC Volts and connect the black lead to black wire pinout and the other lead on your multimeter (usually red) to a yellow wire pinout. The reading on your multimeter should be 12V. Connecting black to black and the red lead to a red wire pinout should give you a reading of 5V.
If fans are in fact turning on, the processor could be bad, as the motherboard needs a working processor installed in order to initiate POST beep codes. There are ways of testing CPUs with a multimeter, but if you look at it and don't see any burn markings on the die or any of the pins, chances are it is ok.
The next component would be the case speaker. It is the internal speaker that will alert you if RAM, video card, or keyboard is missing or bad. There are pins on the motherboard that the speaker connects to. The speaker could be bad or it isn't connected properly. You can buy a replacement. http://amazon.com/o/asin/B002W4M0DW
The RAM, video card, and DVD-ROM player are not important in testing. One or all could be bad, but typically you would get POST error codes that would alert you to this. Lookup the POST error codes for your motherboard from the manufacturer's website.
Hope this helps.