Same thing happened to my 1LT last fall. I picked up a new thermostat from NAPA, some new coolant (50/50 dexcool), followed this post below, and had it changed in my driveway in under an hour.

https://www.chevyhhr.net/forums/how-tutorial-library-21/how-save-$350-00-thermostat-replacement-2-4l-p0128-24493/

FYI my LT is a 2.2 and could follow the instructions just fine.