ORIGINAL: myork
I was trying to read current from one of the 12v sources to ground and I didn't load it first. I just put the red lead of my meter to the +12 and the black lead to ground and BOOM.
myork --
I don't think you damaged your PSU. You can't measure current on this type of supply directly with an ammeter. You can measure the current being drawn by an attached device, however. Your ammeter has a shunt whose resistance is measured in milliohms -- essentially it is a piece of wire with a known and very low resistance and your ammeter quite literally measures the voltage drop across the shunt.
When you touched the meter leads to the output rails, there probably was an arc and pop, then the PSU shutdown. The overload circuitry kicked in -- you PSU treated the shunt as a direct short and cut off. Unplug it, let it sit for a minute, then it should power up again.