Why is My Furnace Not Blowing Hot Air?

This can be due to several issues like 
1. Thermostat related problem
2. Filter dirty Issues
3. Issues with the flame roll out or High Limit Switch 
4. Condensate Line blocked in high efficiency furnaces.
5. Problems at the control board.
6. bad inducer draft motor.
7. Pilot Light don’t stay on.
8. Gas Valve Issues
9. Burner dirty
10. Flame Sensor needs to be cleaned.