Lots of things can damage a lawn. Some of them you've already identified: too much or not enough water, fertilizer, or mowing. Damage also can occur because weed killer has drifted into the area or because of weed competition or insect damage. Disease, excessive shade, or heavy wear and soil compaction can take out a section of your lawn, as well. The first thing to do is pinpoint the cause of the problem. Often, your county extension service is the best place to find out what the problem may be. The experts there will suggest solutions that are best for your region. Once you have identified the problem and corrected it, you have several choices for filling in the bare areas: seeding, sprigging, plugging, or sodding.