If you don't have heat or hot water, the first thing you should do is to call the NYC Emergency Housing Complaints line to file a complaint and request an inspection. You should keep a log of specific dates and times when heat and hot water were not available or adequate. Go to your local hardware store and get a new thermometer that's certified for accuracy. Use it to record the temperature. Save the packaging as proof in Court of valid readings. Case law has established that you may be entitled to a rent abatement, which is a reduction to compensate for lack of services, of as much as 50% of your monthly rental rate. The key to success with this type of claim is detailed documentation. You have to give your Tenants' lawyer ammunition to work with. There is no guarantee of results with these claims - it is largely at the discretion of the Housing Court judge. However, if neighbors participate in supplying evidence of violations, the claim benefits from collective strength and credibility. If enough tenants are adversely affected, you might want to consider a building rent strike.
You can learn more about starting a rent strike here.