Heater blowing cold air?

A couple weeks back, the heater would only blow hot air while the car set the idle. But now only cold air blows through the system. I've checked all the fuses; I've had the coolant system flushed. Replaced the coolant. But since I know nothing about cars, and I don't want to continue guessing through search engines the problem and solution I am reaching now reaching out to anybody that can help.