Ford Repair: running without coolant, coolant additive, coolent

Question
Sam
My daughter mentioned her 2000 Taurus  was overheating and the engine was making a clacking sound.
There were no issues a few days ago, and last year it was flushed and filled, so she figured it was something else. She first noticed it on a long drive home from the lake,then taking it to work next day. She parked it after that and I just got around to looking at it. The coolant res. was almost dry except for some black sludge. I dont know how long it ran dry but it was only driven for a few hours after it began overheating and making noise. I filled the rad and the noise went away after a few minutes and it no longer overheats.
Question: do you think there is any permanant damage? Should I add a coolant additive or oil additive or something? I am not seeing any leaks, so I wonder how it ran dry. Usually coolant doesnt need topped up that often especially here where its cold half the year, so how could it run dry?
Thanks!

Answer
Barry, most likely there is no damage, or minimal to the engine.  As long as the cooling system is full and no over heating has occured.  Then, I would recommend that you change the oil, in case the engine got too hot to effect the oil engine protection property.  As for the coolent being low, it seems the who flushed the system, did not wait long enough to insure that the coolent made its way through the system and fill to the proper level.  Thanks!