If there has been a heat wave, you might have just had a power loss at the time that you first tried to turn on the computer, or a brownout, or something else that caused the psu not to power up the computer. keep in mind that if there isn't enough AC power, the psu won't work as designed.
That said, you might have a dying psu and the heat wave just agravated the problem. As Pengwin wrote, what's you present psu, brand and wattage? Then a better answer might be forthcoming.