My central heat and ac unit is blowing but no heat is coming out the vents, why ?

Sounds like, the igniter not working
Or the pilot light not working,or the gas valve not working or,or,
Maybe the heating elements not working,or bad sequenser, or kicked breaker to the heat kit,or the door switch is open, or broken,

I`m sorry but we need more info, don`t know where to start

What type of system is this------ Heat pump or other?