Whether you need to restore a topped tree or simply clean up one you neglected to prune over the years, getting a messy tree back into shape can take time and a lot of effort. Most importantly, the treatment has to be consistent over at least a few years. You can do this yourself on younger, smaller trees, but for anything that would require a ladder to reach the top of the tree, calling an arborist is a safer move, both for you and the tree because that arborist will be able to shepherd the tree from damaged to divine without missing any steps.
Don't Repeat What You Did Before
First, whether you try this yourself or call an arborist, take note of what caused the tree to look so terrible and vow never to do it again. Obviously, if the damage is due to a storm, this step won't apply. But if you topped the tree, don't do that again. If you neglected the tree, don't ignore it again. This is not something where you can keep doing terrible things to the tree, thinking that you'll just trim it later on and pretty it up.
Try To Open Up the Canopy
Airflow through a tree's canopy is vital for the health of the tree. A lack of airflow means moisture and fungi can take over the canopy, making the tree ill. If you don't spot signs of disease, look for smaller twigs that seem to head off in odd directions compared to the other twigs around them. Focus on areas of the canopy that seem to be thicker or more tangled. The goal is to thin these tangles but not eliminate all twigs in that area. On lower branches that you can reach easily, you can clip these odd twigs using sterilized clippers; cut the twigs above the bud closest to the branch.
For anything that would require standing on a ladder, call an arborist. Don't take the risk yourself.
Look for Leaders
If you've got a topped tree, you'll see water sprouts springing out of the trunk and branches just below where the branch was lopped off. These will be messy and crowded, and you'll need to thin these. But before you do, see which sprouts appear to be the strongest and thickest. Leave those, and remove smaller, weaker ones. It helps to look at a cluster and remove every other sprout if there are a lot. On other sprouts that you're not removing, look at those that have branched out themselves, and remove a few of those twigs (not the entire sprout, but just some of the twigs that grew at the end of that sprout).
The strongest sprouts will become leaders, with one leader per old branch. The leaders should grow and become the new branches on the recovered tree.
Don't whack away at the tree. Be very conservative in what you prune, and remember that you're going to have to do this year after year to train the tree back into good shape. This is why you really want to have an arborist handle this because they can keep records for your trees and pick up where they left off the year before. It can take a few years for a topped tree to start looking normal again, and an arborist will know the most efficient way to restore the tree.
For more information, contact a tree service in your area, such as Phoenix Tree Service.