When heat is on only cool air is blowing out, the fan outside on the unit is not coming on at all what could it be

What type of system do you have? Heat pump, gas package unit, Split system? In a package unit its one big unilt and the heat and air is both in the same unit. In a split system you have a unit outside and an air handler unit somewhere else like maybe in an atic or under the house or a garage or somewhere.

I hope you got this fixed by now