ISNULL() does exactly that:

ISNULL([Field],0)

This returns 0 if [Field] is NULL, otherwise it will return [Field].

But I prefer COALESCE() because it is a little more flexible:

COALESCE([Field],0)

The above code does exactly the same thing as ISNULL(). But COALESCE() can do a little bit more:

COALESCE([Field1],[Field2],[Field3],...,0)

COALESCE() scans through each parameter in order, returning the first non-null one. In this case I added 0 at the end to catch an situation where all of the fields are NULL.