I had my water pump changed about a month ago. This morning, I started my Jeep to let it warm up and when I went to leave it was blowing cold air. I turned off the heat and turned it to cold and then back to hot and it started blowing warm air. I stopped at a local Auto Zone and had them check the coolant and it was empty. How can that be?
The coolant almost certainly wasn't empty, but it was low enough that none could be seen in the header or overflow tank (whichever one it is that they looked in). If you'd actually been out of coolant the car would almost certainly have overheated and there would have been no hot air from the heater, since there was no water flowing through it.
As to how the coolant got down as low as it did, there are two likely possibilities:
You've got a leak, probably relatively small since the work was done a month ago, or
There was some air trapped in the cooling system that finally worked its way to the header/overflow tank.
Keep an eye on the level in the tank and also pay attention when you park to look for possible drips. Hopefully it was just air working its way out and the level won't go down again.