A neglected lawn can be brought back to a reasonable condition by mowing and feeding, weed and moss control, as described in Mowing, Feeding, Weeds and Moss.
A raised bump skinned by the mower
If a lawn was laid level originally, and the level is still good, it is not worth digging it up to make a new lawn. It can be restored. Clear any debris. Use a rotary mower without a grass-bag, or a strimmer, to cut the heavy growth. Rake off the mown grass. Tear out old grass and moss with a rake. Mow again. Apply a spring lawn feed.
Begin regular mowing. Apply lawn weedkiller two or three weeks after feeding in spring. A repeat application of lawn weedkiller might be necessary, and feeding should also be repeated. Moss control may be necessary. Top-dressing and re-seeding overall will help to restore the sward. Then simply maintain routine lawn care.
Bare patches are generally caused by wear, disease, dogs, petrol spillage or they can be caused when weeds and moss are killed off. In a new lawn, uneven sowing or bad germination may leave bare spots.
Lifting lawn sods to remove a bump
In March/April, or September/October, fork the soil surface lightly until the top 5 centimetres is loose. Add a thin layer of moist peat and/or fine soil. Level this out. Sow seed at 30 grams per square metre. Criss-cross the patch with string on pegs, to give it a chance to recover. Thin patches can be treated in the same way.
To repair bumps and hollows, cut and lift the sod. Add, or take away soil, as necessary. Replace the sod and firm it down. The sod re-establishes better if this is done in autumn. For minor hollows, the top-dressing technique described under Feeding is an adequate solution. Repeated over a few years, the hollows will fill up.