We can't make a concrete diagnosis from the photos alone but suggest you hire a certified arborist who can evaluate the tree in person if you are interested in knowing what potential causes contributed to decline. Some arborists are employed by tree-care companies but others consult independently. They cannot cure every ailment but can help make a diagnosis as well as look for sources of tree stress.

Trees with advanced decline are not salvageable, though, so while there is a chance it could be stabilized in the sense of halting further decline, it would otherwise need to be replaced it you wanted it to look like its former, fuller self. The arborist can also help ID the tree if you don't know what kind it is; it looks like a flowering cherry or maybe a crabapple to us, but it's hard to see enough features to be certain. Identification is important because different tree species are vulnerable to different pests, diseases, and environmental stresses. Usually, a pest outbreak like wood-boring insects or wood decay is preceded by injury to the bark or stress from weather, such as drought or overly-wet soil. (Cherries, as an example, have little tolerance for soils that become too wet.)

The gray-green growths on the bark are lichen, a harmless fungus-like organism that can appear to be associated with tree decline but is not the cause of it; instead, the lichen only benefits from the increased light and moisture access as the tree canopy above it thins out over time. Wood decay fungi can produce mushrooms emerging from the bark, but they do not look like lichen.

Miri