Root problem is my guess, but which one ?
A severe gilrding root could do that. Or a strike from a fungus like Armillaria mellea.
The layer of something on the ground is way too thick and the trees usually don't like to have their foot buried.
The tree is already lost, no hope, but you can dig out the top or the root system to look at what's going on.
Here, many mapples are killed by the mapple soot, an agressive parasitic fungus on the trunk and limbs. The very thin surface layer of the bark peels off and reveals a black/dark brown dusty coat. One or two years are enought to loose the tree. Solme can hold a little longer, but they are condamned. The dark dust is a very nasty stuff for our lungs.