Okay, so you ruled out the HDMI cable (but not the HDMI monitor).
There are three alternate reasons why your VGA output may not work on the computer (assuming it's got a VGA port to attach to).
1) The VGA port is on the built-in, integrated video included on the motherboard and is being overridden by an added-on "daughter" video card that has HDMI on it.
2) If both the VGA port and HDMI are supplied by the same card, either the motherboard or a daughtercard, then a BIOS or firmware setting is disabling the VGA in favor of the HDMI
3) Your VGA port or the VGA cable you are using is broken (very unlikely).
#1 above is the most likely. Ironically, you can often fix #1 by changing a BIOS setting, as in #2. But if you do have an add-on video card, it is often easiest and most conclusive to simply pull it out of the system so that the motherboard's VGA port is absolutely the only video-out device on the system.