The most influential theory about the collapse of the Western Christian world was that the Germanic tribes delivered the death blow to a civilization already weakened by invasions, decadence, warfare with invaders, rival would-be rulers within the Empire, and Christianity.
At least, that's the standard history that most people learned when I was young (Pre-PC). It was a typical distillation of Edward Gibbon's Decline and Fall of the Roman Empire books.
I just heard today of another theory, from Declination blog, the Pirenne Thesis. It was Pirenne's suggestion that, although the Germanic tribes sliced off the top level of governance, much of the basic structure remained, and that the Germans had largely been assimilated and absorbed into the Latin world.
What Pirenne thought dealt the death blow to Western civilization was the invasion of Islam. There's a fairly long pdf at the link that explains his thinking better than I could.