The easiest way to rectify any low spots in your yard is by filling the sunken area with topdressing and compacting it to ensure it is firm. Make sure you reach the level of the surrounding areas and add some new grass seed to foster the re-growth of your lawn. After you have filled the low spot to a level point and added the seed, you will need to water generously. This will help the rooting system take effect. In time, the patch will be gone. You’ll then have a healthy spot of new lawn to match the areas around it!