Is your lawn feeling a bit spongey? If so, this is called thatching and its actually quite easy to fix!
You'll simply need to scarify your lawn. This is where you cut it very low, back almost to the runners. You may have to do it in a couple of mowings over a few days reducing the mower height each time.
Just make sure your mower blades are sharp before you start as that will make the job a lot easier and ensure that your mower is cutting the leaf rather than tearing it.
Once you're done, give it a good fertilise and plenty of water until it recovers. You should find the sponginess will now be gone!