A power supply should last considerably longer than 300 days. I've had mine for 3+ years now. Still, this would also be my first guess given the information supplied.
Advanced Tip:
That said, the easy way to test a power supply is to disconnect all of it's cables, then plug in the power cord. Take the motherboard connector (the big one with 20-24 pins) and use either needle-nose pliers, tweezers, or a paperclip to touch the green pin to any of the black ones. This simulates a "power on" button.
If it is working, the fan of the power supply should spin up.
Advanced Tip:
That said, the easy way to test a power supply is to disconnect all of it's cables, then plug in the power cord. Take the motherboard connector (the big one with 20-24 pins) and use either needle-nose pliers, tweezers, or a paperclip to touch the green pin to any of the black ones. This simulates a "power on" button.
If it is working, the fan of the power supply should spin up.
