My heater don't work in my 05 cobalt

First check the simplest solution - make sure you have coolant topped off and the thermostat is working. Low coolant or a bad thermostat seem to be the most common reasons why a heater stops putting out heat.

If the blower fan is not working, and especially if it quite gradually, losing one speed at a time till they were all gone it will be the resistor in the ductwork.

While changing the battery the trunk was accidentally lock. The trunk release won't work. How can I get in the trunk?

I presume you've tried the key in the lock on the trunk lid. I know most of them are seized up from lack of use. You could try hooking power to the power points under the hood where you would normally hook up to boost the car, but if the battery is disconnected I'm not sure that would create a complete power circuit for the power release. You could also try posting a new question thread on this site instead of using an ole one. I'm likely the only one who will ever read your question on this old thread.