Short answers, pulled from the story.
Edward Gibbon was the first to produce a well-researched and well-referenced account of the fall of the Western Roman Empire, in his book The History of the Decline and Fall of the Roman Empire, published beginning in 1776. He dated Rome's decline to 180 and identified moral decay, Christianity, militarism, and universal domination as the primary causes.
In 1984, historian Alexander Demandt enumerated 210 different theories for the fall of the Western Roman Empire, and new theories have emerged since. The large number reflects both the significance of the question and the scarcity of surviving evidence from the 4th and 5th centuries.
The traditional date is the 4th of September 476, when Romulus Augustus, the last Western Roman Emperor, was deposed by the Germanic chieftain Odoacer. Gibbon chose this date as a convenient marker, though some historians note that Julius Nepos, recognized as Western Emperor by Constantinople, continued to rule in Dalmatia until he was assassinated in 480.
The Pirenne Thesis, developed by Henri Pirenne and published in the 1920s, argues that the barbarian invasions did not end Roman civilization because the invaders wanted to preserve Roman ways of life. Pirenne held that the real break came in the 7th and 8th centuries when Arab conquests severed Mediterranean trade routes, cutting western Europe off from commerce and reducing it to subsistence-level agriculture by the time of Charlemagne.
Historian William H. McNeill argued in Plagues and Peoples (1976) that epidemic waves beginning around 165, possibly the first outbreaks of smallpox and measles in the Empire, killed roughly half the population over about twenty years. McNeill concluded that this population loss left the state apparatus and army too large for the surviving population to support, accelerating economic and social decline that ultimately killed the Western Empire.
In The Collapse of Complex Societies (1988), anthropologist Joseph Tainter argued that Rome collapsed due to diminishing returns on investments in social complexity. As the cost of maintaining bureaucracy, garrisons, and civil government grew, each new problem required greater resources to solve. When invasions and crop failures converged, there was no additional territory to absorb the stress, and the empire fragmented into smaller units.