score:10

Accepted answer 
Enumerable.OrderByDescending

if the problem was that you wanted descending and not ascending

score:5

If you mean a non-generic IEnumerable, you should use Cast or OfType to get an IEnumerable<T> first, then you can use the normal OrderBy / OrderByDescending calls.

For example:

IEnumerable test = new string[] { "abc", "x", "y", "def" };
IEnumerable<string> orderedByLength = test.Cast<string>()
                                          .OrderBy(x => x.Length);

You can also do this by explicitly stating the type in a query expression:

IEnumerable<string> orderedByLength = from string x in test
                                      orderby x.Length
                                      select x;

EDIT: Now that the question has been clarified, the query expression form is:

var query = from value in collection
            orderby value.SomeProperty descending
            select value;

score:4

If your talking about a generic IEnumerable, below is a trimmed down example of usage.

// Using complex type
class Person()
{
    public string Name;
}

IEnumerable<Person> myEnumerable = new List<Person>();
this.myEnumerable.OrderByDescending(person => person.Name)

// Using value type
IEnumerable<int> ints = new List<int>();
ints.OrderByDescending(x => x);

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