You only show one foreach loop, so it's got to be one of these lines:
for (int idx = 0; idx < xmlFiles.Length; idx++)
XDocument doc = XDocument.Load(file_name);
foreach (XElement data in doc.Descendants("VehicleBreadcrumb"))
In all cases start with the debugger:
In the first case, check the value of
xmlFiles.Length
If it's zero, then ...
In the second case, check that the XML Load is returning in a timely fashion: if the file is too big, then it will take considerable time to load and it may be they you just aren't waiting long enough.
In the third case, check there are any Descendants with the right key. If there aren't any being found, then ...
We can't do any of that for you - we don't have access to your data or your code while it is running, and you need them both to start diagnosing the problem.