1. Power supply doesn't work or PC doesn't start up
The power switch cable from the PC case is not connected to the mainboard.
The 20/24 pin ATX connector is not properly attached and doesn't have proper contacts.
The P4 connector is not plugged in (presuming the mainboard requires it).
The CPU is not properly attached to the socket. Memory is not compatible with the mainboard. (PCs can start without memory chips; if the mainboard produces post codes (beeps) then the power supply is OK).
Check that all PC cards are correctly seated into their slots.
The power supply mains switch is not switched on.
Power mains cable is not connected to the power supply unit.
The multiple socket outlet may be defective, or need to be switched on, if a switch is available.
The mainboard may be defective.
2. There is a short-circuit in the power supply
Insufficient cooling of the PC case; additional fans can remedy this.
The PC has been working non-stop for months without turning off the system at any point in the interim.
The power supply unit is being "overworked" by a retro-fitted high-end component (e.g. graphics card). This oversteps the power supply units power rating.
Lightning strike on the power network (overvoltage). The bus bar for the floppy disk drive has been connected the wrong way round, causing short circuit (many cables are burnt out).
The PC should be cleaned up (hoovered) of dust from time to time, particularly the fans around the power supply, CPU and graphics cards. An excess of dust on those components leads to heightened temperatures, which can lead to failures. Dust can become statically charged and as such electrically conductive, which can also cause the computer to crash.
3. The power supply fan is too noisy
The temperature in the PC case is elevated, because there are no or too few fans mounted in the case for cooling
The CPU fan is blowing its warm air directly into the lower power supply fans because an insufficient gap was left between the two and thus the temperature inside the power supple is increased. In such a case the fan regulator reacts by increasing rotational speed for all attached fans, which in turn leads to a high level of noise.
4. Device makes a (chirping) noise
The power supply is potentially being "permanently overworked" by a retro-fitted high-end component which is completely exhausting available capacity. This leads to elevated temperatures in the power supply, which causes the fan to rotate quicker, which generates more noise.
The chirping noise can also come through the coils: on some mainboards coils whose ferrite cup coils are only stuck on may also chirp because of vibrating caused by magnetic forces. This is what produces the chirping sound.
5. Note: if you are looking into a ENERMAX power supply through the fan rotors,
you may see special white tape on various coils. This is normal. They are a necessary remnant from the production process and do not negatively impact performance in any way.