heat do not work,it blows cold air,whats the problem?

There are a lot of potential issues here. One is that you are low on coolant or your heater core is blocked. Check to make sure you have sufficient coolant. Two, there is a valve called the heater control valve which is vacuum-operated. All of the heater lines pass through here and it meters the coolant going to the heater core so that you get warm air when the operating temp in the coolant is correct. This is a $20 part and can cause a problem. Another possibility is that the blend door in the climate control system is not operating and is blocking off warm air from entering the vehicle. You'll probably need to have a mechanic look at this to get more information for you.

heater core is probably bad