Goodman Furnace model PGB048075-1
Is it possible that a bad high limit switch that causes the fan to constantly blow whenever power is applied. The coil was clog and air couldn't get through. I cleaned the coil and now I want to turn the furnace back on and see if it trips the limit switch now. Could you tell me how to turn the unit back on, cause our thermostat inside is not on unless the unit is on.

"thermostat inside is not on unless the unit is on."
It's the other way around, when the thermostat is on, it will turn on your Unit(s).

Just turn the power to the furnace back on, and then turn the stat up.