You need to make a complaint to your local housing, building or health official responsible for overseeing the condition of rental housing. Assuming the official finds it to be the case, that official will notify your landlord in writing that the lack of cooling materially affects the health or safety of an ordinary tenant. Your landlord will then have a period of time to make the necessary repair, usually three days. If the landlord does not complete the repair or submit the necessary petition to get more time to make the repair, that opens up the "repair and deduct" remedy for the tenant (fix it yourself and deduct the cost of repair from your rent).