Explain the fall of western rome?

Fixing the year 476 aD as a point is too simple. You will find historians who say that Rome really fell in the 7th Century (see Henry Pirenne's "Muhamad and Charlemagne") and others like Mary Beard (see her recent "S.P.Q.R.) who say that the western empire was completely changed when Caracalla (c. 212 aD) gave citizenship to all Roman provincials, thus destroying the old sense of "being Roman".
In fact, migratory waves beginning by the times of Marcus Aurelius (c. 180 aD) pressed the frontiers, and the only possible answer Romans could give was a bigger army. To get these men, foreigners (or, you could say "barbarians") had to be recruited as mercenaries, and by the 3rd century aD most of the army did not even speak Latin.
So, the best thing to do is to say that the fall was a long and gradual process.