I have a 1990 Grand Prix and it says i have low coolant but there is plenty of coolent in it and my temp gauge is telling me that its over-heating and my heat blows out cold air.Ive changed the thermostat and i

If you didn't bleed the air from the cooling system after changing the t-stat, that is your problem.

Well i changed it 3 months ago and my heat worked up till about a week ago

you may have a coolant leak somewhere. The lack of heat, temp reading high and low coolant light on all indicate air in the system.

ok if there is air in the system how do i go about fixing this problem

You need to pressurize the cooling system, and listen for where the hissing is coming from. Common coolant leak area on your car is the "quick connect" nipple on the drivers side end of the intake near the t-stat housing.

^^^haha he said nipple....j/k