My wife was working on her windows based PC when suddenly it started making what she described as a weird beeping. She shut it down and tried to restart, but the beeping was constant at boot, so she powered it off and left it for me.
Having recently tinkered with this PC (added an SSD and upgrade the RAM) I was familiar with the insides. I immediately reseated the RAM, reset the CMOS. Power on, same constant beeping.
I looked up the post codes and the closest was 4-4-4, which is cache error, but this seems much faster than any post code I had heard before. So I turned it on, and let it keep booting -- and it actually did boot! Kept beeping though.
To make a long story short, I finally figured out there was a key stuck down on the keyboard.