Answer: Since you don't know what may be causing the holes you may just want to fill them with soil from other parts of your yard, then rake out the dead grass and patchy areas, broadcast grass seeds over your entire lawn, lay on a light layer of peat moss (to help keep the grass seeds moist) and then water well as often as necessary to keep the seeds moist. New grass seeds usually germinates in 7-10 days and can be mowed when you regularly mow your lawn. If the holes reappear you should be able to determine whether you have gophers, moles or snakes by the way the soil is mounded (or flared out rather than mounded. Gophers and moles leave pyramid shaped mounds of soil when the push the soil out of their tunnels; snakes tend to move the soil back and forth as they enter the soil. Gophers and moles can be killed with scissors-type traps; snakes will just have to be disturbed often enough to make them want to hole-up elsewhere.
Best wishes with your lawn!
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