Yep! I recommend you to start from scratch. Here is what you should do:
Step 1: Kill the bad lawn by spraying weed killer. After a week, spray again should small weeds started to grow.
Step 2: In 5 days or a week, mow the dead grass at their maximum short length and remove the clippings.
Step 3: Use a dethatcher and run over the dead lawn in two directions. Make sure that the machine runs low for the blades come in contact with the soil.
Step 4: Rake up the unwanted grass and weeds then slather a layer of lawn soil to areas that are too low or over excavated.
Step 5: After a day, seed your lawn with a product of your choice. Seeders vary according to the site of the grass. The grassed area may be a full-sun, shade tolerant, heat tolerant, and even encountering heavy traffic like car loads or pedestrian traffic.
Step 6: Distribute the fertiliser on the soil the same day when you seeded it over.
Step 7: Start watering it a couple of times per day for a number of weeks.
Step 8: Carefully mow the brand new lawn. The recommended mowing height is approximately 2 and a half inches.
Step 9: After a month after you seeded the lawn, drizzle your lawn with a é¼*awn food to enhance its ability to absorb water and nutrients, prohibit weed growth as well as protect it from future lawn issues.
ENJOY YOUR NEW LAWN!! I hope this helps.