I have a 2001 chevy Impala , in the past week the heat goes in and out now it just blows out cold air and doesn't defrost windows or anything. Ive had the thermostat changed and that wasn't it

Hearer core could be plugged. When you had your thermostat changed did they "burp" the air out of the cooling system with the bleeder valve? The blend door or blend door motor could be broke, wore out or stuck.

Can we assume that whoever replace the thermostat also topped off the coolant levels? Does the temp gauge on your dash move? Does the engine operate normally without overheating? The first thing you can check yourself is that you have proper level of coolant in the reservoir bottle. If that's okay, then you either have 1) restricted coolant flow, limiting what can reach the heater core 2) a bad heater core 3) a bad/stuck temperature blend door. #1 can be caused by several issues, bad water pump, bad radiator, blockage, etc. #2 a bad heater core would produce no heat. #3 a bad/stuck temp blend door would prevent the airflow from the blower from blowing across a heater core. The only way to know which is the problem is to troubleshoot, or take it back to whoever changed the thermostat and ask them what's going on.