First of all - don't panic. Your grass might not be dead, it may just be dormant. Maybe it is stressed, and we need to find the reason.
How long has the area been grass - is it only recently sown? What was the weather like where you are during the last winter? How much rain have you had this spring - say in the past two months?
When you cut it, how low do you have the blades on your mower?
Maybe with a few more clues, we can work out what's going on.....better than just guessing