What do three long beeps when booting a PC mean?

I have a custom built PC. It’s been working fine for three years, with only one or two minor problems. i shut it down last night and didn’t touch it until this afternoon. When I pressed the power button, The LEDs in the computer came on, all the fans spun… but the keyboard and monitors did not light up and I heard three long beeps from the system. In the background i can hear harddrive activity that is consistent with Windows 7 going though it’s boot process, but there is still nothing from the monitors or the keyboard. The power button acts as if Windows has taken control of it, so I hold it down to power off, then turn it back on and get the same response. Three long beeps.

I have not adjusted the hardware in the last three months since i replaced the power supply. Does anyone know what is happening? Thank you.