Some interesting answers.....
There is no reset button by the fuse panel, for the low coolant light....that reset button is for resetting the oil change light, and in some cases the low tire light....
What has happened, is what sowseyed505 said....the float is stuck in the sensor.....however, this only happens when reservoir gets really low....just adding coolant doesn't "unstick" it.....take the end of a screwdriver handle and tap the reservoir in the area of the sensor(don't hit the sensor itself)....the light usually goes out....
If the reservoir was NEVER low, then you could have a bad sensor or connection as xeroinfinity stated....disconnecting the sensor should make the light go out(this checks the circuit).......