Dig a trench underneath the fence along the length of the gap you want to close. Use a spade and a shovel to dig the trench.
Fill up the trench with large stones and bricks. Cover the stones with the soil you have removed from the trench.
Attach chicken wire that is twice as wide as the gap to the bottom of the fence. Fold it in half and join the other end back onto the fence.
Put some additional rocks onto the chicken wire to make it stay firmly in place. Hide the rocks with the remaining soil.
Put chain-link fencing onto the ground near the fence. Attach the end of the chain link to the existing fence. This will prevent animals from reopening the gap.