It wouldn't hurt to hire someone to tell you exactly what is going on with your lawn (disease, insects, etc.) They will most likely provide you with enough information as to allow you to go from there on your own.
Bring in a professional to make sure your lawn is diagnosed properly otherwise you'll spend a lot of money trying to find the right solution. Then treat the lawn with chemicals on the market. Then you have to go through the process of fertilizing and feeding to bring it back.
It all depends on the disease. You can go to the nursery and describe what the disease looks like and maybe they can help you. Some diseases you need to dig out the patch and apply new seed. These jobs are better done in the fall or early spring, in the summer its to hot.
