This can also be caused by a faulty USB device. Unplug all USB devices including any hubs and reboot. If the computer boots, plug each item back in the same places one at a time, starting with hubs, and reboot after you plug in each one. If one of the reboots fails, then the last device you plugged in will be the culprit. This is a common symptom when the receivers for wireless mice/keyboards fail.
Also, what brought about the forced shutdown in the first place? Was this due to a lockup or other problem, or were you running late for a party?
In regard to the BIOS beep, you probably know that the single short beep is the OK code from the POST test. The fact that you weren't getting this, means that the POST either didn't start, or didn't complete. When you removed the memory and triggered the memory error code, this indicates that the POST is in fact starting. You therefore know that there is a hardware issue that is preventing the POST from completing. You'll have to use a process of elimination to find it, or a good POST analyser that plugs into the board may help.
Another common cause for non-booting / non-POST is a faulty network card. The real problem here is when you have an onboard LAN which fails and you can't get into the BIOS to disable it. Personally, I never use onboard LAN ports for this very reason, and disable them as soon as the board is installed. The mains power around here is woeful, thus power supplies and NIC cards are common failures. The price you pay for putting power cables undergound I guess - no maintainence.