mechanic removed the computer under dash because he said that was why the check engine light was on and when he replaced it with another 1 i have no heat nor air how can this be fixed?

Check relay and or the fuses need checked for blown fuses or switch went bad or the grounds for it are bad or loose or The one he replaced it with had those issue so either get another used one or put the old one back in and live with light on or what I would do first is try the old one again and see if those things start to work again if so you know it was the computer he put and if not you no you need to look elsewhere.