GM-GMC: no heat

Question
2011 GMC Equinox LT, 4cyl, 2wd, 32,000 miles. Heat worked great, no problem, the next day I have no heat. I checked the heater core, and 'water' is running through it. Blower is working fine, the AC and the HEAT is blowing out the same cold air. Please help.

Answer
these are the things i would check

1)   Low coolant level on a vehicle due to a coolant leak.
2)   Thermostat that is stuck open. <<< COMMON
3)   Plugged or leaking heater core.
4)   Heater blower that is not functioning.
5)   Blend doors that are not functioning properly.<<< COMMON