Front heat....

Like the others said, check the coolant level. Some systems may need to be burped when you fill it back up too to get an air lock out. Beyond that, the blocked heater core but that's a long shot. Also make sure the doors and baffles are actually opening to allow air flow to/from the heater core. Also if there's a valve that regulates the flow to the heater core make sure it's operating.

Low coolant or blocked heater core.

Low on coolant would be the first thing to check.

Chassis make, model and year?

What chassis is it?

Do you mean through the defroster/heater system, is it just blowing cold air? If so, possibly you are low on engine coolant or engine thermostat is bad. I'm assuming you have a gas engine. Or if no air flow, maybe blower motor.