2003 Pontiac Sunfire Engine temperature/Heater does not wor

I was driving last night about 30 mins on a highway. When I exited the highway I noticed my heater was no longer working (blowing out cold air) and my heat
gage was almost in the "red". The check engine light did not turn on. After inspecting under the hood I
found there to be plenty of coolant, the engine did
not seem hot (no smoke or heat radiating).
Additionally I did take notice to the serpentine belt and thought that it may be a bit loose. Do you have any ideas of what the underlying issue may be?