Fatigue from the brain down and the muscle up
Exercise fatigue is not a single thing. It is usefully split into central and peripheral components. Central fatigue refers to a reduction in the drive the nervous system sends to muscle, so the brain and spinal cord recruit fewer motor units or fire them less vigorously. Peripheral fatigue happens at and below the level of the muscle itself, where the machinery of contraction becomes less effective despite adequate neural commands.
In most real efforts both contribute at once, and their relative weight shifts with the type and duration of the task.
Where the contraction actually falters
Peripheral fatigue involves disruptions such as impaired calcium handling within the muscle fiber, accumulation of metabolic byproducts, and depletion of energy substrates, all of which reduce the force a fiber can generate. Central fatigue can stem from changes in brain chemistry, feedback from working muscles, and the cumulative effort of sustaining drive. The classic lactic acid as the cause story is outdated; the metabolic picture is more complex.
This is general educational physiology and not medical advice. Persistent or unexplained fatigue should be evaluated by a qualified healthcare professional.