Blows only cold air

Low coolant, bad thermostat, bad heater control valve, plugged heater core, defective blend door control or door are the list of items that can cause low/no heat.

Check the coolant level.
Once the engine warms up touch the heater hoses, they both should be hot. If neither one is hot you have a heater core issue. If both are hot but you get no heat then check the blend door for proper operation. (Turn the controls from full cold to full hot a few times and listen for the doors operation)