I have a pontiac sunfire(1999). All of a sudden the coolant leaked out and the car over-heated. When I looked at it a hose which is attached up by the coolant cap was not attached to anything (not sure if that is the way it is suppose to be). The hose to the engine and the rad are secure. I opened the cap and nearly all the coolant is gone. Should I buy more coolant and fill it up? Why would this do this?

YES!

The sensor in the tank is probably bad. If you overfill the tank it will come out the overflow when your car warms up. You may also have a leak in a hose or radiator tank.

Spray clean engine and check for leak - it could be a hose, waterpump or head gasket.

I would start by replacing that section of hose. Depending on how much it's leaking it could be the source of the engine overheating as well. After it replaced make sure coolant level is where it's supposed to be. Hope this helps

Check your coolant level in the coolant overflow bottle and in the radiator. If it is low, check the upper and lower coolant flow hoses and overflow hoses clamps for leaks. Check beneath the car for signs of coolant.

A thermostat that does not open and close would cause these very symptoms. Since it does not allow coolant to flow through the system, it will make the engine overheat. Since it won't allow the "heated" coolant to pass by your fan, the heat will continually blow cold air.
Stop driving the car immediately, becuase the next step will be some blown gaskets - especially head gasket. Other possible reasons can be as follows:-- It could be just low on coolant. if it is then add coolant... with low coolant the water pump can not move enough coolant to supply hot coolant to the heating system . also if its low the motor will over heat fast..... If the system is full then you may have other problems.. such as 1) bad thermostat 2) bad radiator cap ( not holding pressure so it will leak coolant) 3) blown head gasket . 4) bad heater core (leaking coolant out.. check the floor board under the dash on the passenger side.) 5) bad hose or clamp. Thanks. Keep updated for any more query. You can rate this solution and show your appreciation.

make sure you have radiator full of coolant
run the car to operating temperature
check the operation of the cooling fan
feel the upper radiator hose and the lower radiator hose. they both should be hot.
if one is cold then you may have a bad thermostat or restricted flow
if cooling fan does not come on when engine is hot then you have bad cooling fan switch, relay or motor.
good luck

there is a steel pipe that runs across the motor there, they are prone to corroding and leaking, you might get away with cutting it and bridging with a piece of hose but the best bet is to replace it. We have been lucky a couple of times and found a car in the wreckers that had newish ones fitted.

There could be a leak, to find out you must pressure test the cooling system. Also test the cap.

you should look good around the radiator while the car is running for a while. then check the bottom of the overfill tank. If you can not find a leak ther then you can look at the hoses. dry everything up first let the car run to pressurize the radiator. I hope this helps