Chrysler Repair: 2003 Chrysler 300M Overheating, coolant level, head gasket

Question
Last night my 300M suddenly overheated and the temp light/alarm went off.  
I had only driven 4 miles and the temp gauge was in the red.  I immediately
pulled off of the road. It was dark so I could not see if the coolant level was
at the proper level.  I waited a while for my Dad to bring a flashlight, etc.  
When he got there we saw that the coolant was VERY low but there was no
signs that anything leaked out after I stopped.  We added water and I drove it
the short distance home with the heat blasting.  The thermostat went to the
mid level and when I turned into the driveway it spiked again to the red
before I quickly shut off the engine.  I have never had any problems like this
with this car and I had no signs until last night that something was wrong.  
Why would all of the coolant suddenly drain out?  Aren't leaks usually slow
and would it not be evident under the car?  Please help.  I have to take it for
repairs and my Dad and I have very limited knowledge.

Answer
if u don't see a leak    would want to pressure test the system so have it under pressure to see if there is an external leak or  possible head gasket blown  pushing out the coolant out the over flow tank.
Possible fans locked up,head gasket,water pump,thermostat.