What are the reasons for the decline of trade in western europe?

I'm assuming you are referring to the Dark Ages and not today. Western Europe was shut into isolationism after the fall of the Roman Empire. The Roman Empire ruled over Western Europe for 1,000 years and it collapsed, leaving Western Europe confused and disadvantaged. The fall of Rome was brought upon and succeeded by Barbarians, who were terrifying people. After the fall of Rome, Europe was shut into darkness, where Europeans were basically living and dying within a few miles of where they were born. It wouldn't be until the voyage of Marco Polo that Europe would flourish once again.