2002 Ford Explorer No heat from vents

This is a secondary car for me, and I drove it for the first time in a month. Since it was cold outside, I assumed that the heat would kick on when the engine warmed up enough. The gauges read that the engine was warm (in the middle of the gauge, so normal) but no heat.

I jacked up the temp on the controls and still nothing. Could this be a thermostat issue or a heater core issue (or something else). This is the first time this has ever occurred.