First thing I'd check is that your RAM stick is in the correct slot. Make sure it's in the first slot, and not the second. Next, make sure that it's seated all the way. Next, check the connection for your CPU and the CPU fan. If the fan is not spinning, touch the heatsink while you turn it on to see if it warms up (should heat up rather quickly). If it doesn't heat up, then either your RAM is bad (or incompatible), your cpu is bad, or there is a problem with the power connection from your power supply to your motherboard. I haven't seen the AmigaOne yet, but check for a 4-pin mobo connector (common feature on p4 motherboards).
If the light is on for your motherboard, and if you turn on the power supply, and you can hear it running, then most likely it is not the power supply. Try plugging your computer into the power supply, turn on the power supply, and move around the connector to see if you can get anything to power up.
