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That beep code, if the power light also blinks 4 times, means that the power supply has a defect, generally speaking.
A. the power supply might be overloaded? unplug all USB devices, power-reset the CPU for at least 5 minutes, then try it again.
B. if you put any extra device, like an additional hard disk, into the machine recently, it might be causing the problem,
C. the power supply connectors may be obscured with dust, or need re-seating (which should only be done if you are familiar with anti-static precautions and CPU disassembly/reassembly.
D. the power supply might be plugged in wrongly or loosely? try unplugging the power cord - at both ends - and then re-plugging it (and into a different outlet, at the power outlet end.)