It is virtually impossible to determine the cause of the problem you are experiencing with your lawn without being able to see it firsthand. There are a number of possible causes for a lawn to yellow and look unhealthy: fungus/disease, the need for a fertilizer, lack of watering or insect damage.
The browning/yellowing may even be due to two or more of these problems aggravating one another. Yellowing can be an indication of a phosphorus or iron deficiency.
You would need to have a soil test done to see which nutrient is lacking in the lawn. If it is a phosphorus deficiency, we would recommend applying starter fertilizer. This special fertilizer is high in phosphorus, which in turn promotes strong root development. If it is an iron deficiency (particularly with bentgrass lawns), then apply turf builder with Iron.
Consult the problem solving areas of this web site for help, and view possible answers to your problem.