Check the level before driving it. Open the cap, you'll see the 2 wires for the coolant sensor, if they're not really fully submerged, add more coolant/distilled water.
Don't use tap water.
If it's CLOSE and you're int he south, you'll probably be fine with just adding distilled water to save some $. But if it gets low again, get some coolant. (G12 from the dealership or worldimpex.com or something, not the store-bought stuff)
There's a CHANCE it's not just low coolant. But that's very likely the problem.
worst case scenario, your water pump blew up and is no longer circulating coolant... but I bet it's just the sensor getting a bad reading.