It is normal on many trees for the lower branches to eventually get shaded out as the crown of the tree fills in above them but it sounds like this happened more abruptly on your tree. The first thing to check is to make sure that mulch hasn’t been piled up volcano-like up against the bark of the tree.  Piling mulch against the bark stifles the flow of moisture and nutrients up through the bark to the branches and leaves above.  It is like slowly strangling the tree to death. It may take years but volcano mulching is a tree killer.  Brush any mulch or dirt away from the trunk of the tree until you find the original soil level and keep it that way with the mulch pulled away from the bark a few inches so the bark can “breathe.”

It is also possible that the tree has a problem with borers…insects that “bore” through the bark and then eat away at the nutritious layer just underneath the outer bark layer. Look for the tiny pinhole-sized entry points on the bark.  If you find those, then you’ll need to treat the tree with a systemic insecticide that you’ll pour into the soil at the base of the tree.  The tree will take up the insecticide through the roots and pump it throughout the tree.  Any insects that are feeding under the bark or on any part of the tree will be killed.  The systemic insecticide lasts long enough that a single treatment is all that will be needed.  Always read and follow the directions on the label to the letter.  Here’s a link to more about that product:  https://www.bonide.com/products/insect-control/view/609/annual-tree-and-shrub-conc