Reformed theology does not use that phrase, "Natural Law" but this is what it says about "the light of nature, and the works of creation and providence", which may be the closest to what is being asked about here. I quote from a Reformed book for study classes examining the Westminster Confession of Faith. Its first chapter deals with the Holy Scriptures, and what this reveals to humanity about God, compared with the limited understanding given through 'nature'. 1.1 of the Confession teaches us:
"(1) that God reveals himself in two distinct ways to man: in nature
and in Scripture, (2) that no man can evade constant confrontation by
that which reveals the living and true God (even without Scripture),
(3) that all men are without excuse for their ignorant and sinful
condition, and (4) that Scripture is necessary for true and saving
knowledge of God because therein alone is revealed God's redemptive
provision.
It has long been the habit among Christians (even of Reformed
persuasion) to speak of the insufficiency of natural revelation, as if
there were something defective in the revelation it makes of God. This
may be seen in the traditional use of the 'theistic proofs.'" The
Westminster Confession of Faith for Study Classes, page 1, G.I.
Williamson, Presbyterian and Reformed Publishing, 1964
I will not go further into that last point, but extract further bits relevant to the actual question here. Suffice to say that it shows how in the scheme of those 'theistic proofs'
"...the creature fixes the terms under which God must present his
credentials... What is wrong with such an approach? Simply this: every
fact (and the sum total of all facts) proves the existence of the
God of the Bible. ...'The heavens declare the glory of God; and the
firmament sheweth his handiwork... There is no speech nor language
where their voice is not heard' (Ps. 19:1,3)...
Man was once God's true image. He alone among the creatures could
think thoughts of his Creator... Nature revealed all that Adam needed
for a right knowledge of the nature of God, and the world. But how
could Adam know the will or purpose of God? And how could he know what
his own will and purpose ought to be? The answer is: only by special
(word) revelation..." [Ibid. pp. 1-2]
It then shows how the sinless Adam, being the image of God, was wholly dependent on God, but when striking out on a course of disobedient independence, violated that dependent relationship:
"He is metaphysically God's image, although he is ethically God's
likeness no longer. Man's determination to be independent of God is
doomed to frustration, and he is clearly and constantly reminded of
this through natural revelation. Natural revelation never ceases to
declare to sinful man the fact that the true God is, and that man's
very existence is wholly dependent upon God. In order to continue in
rebellion against God, therefore, a man can only lie to himself about
the situation. He must suppress the truth in unrighteousness (Rom.
1:20). But this suppression of the truth (whereby sinful men refuse to
know either themselves or the true God aright) is wholly due to sin,
and not in any way due to an insufficiency or defect in natural
revelation." [Ibid. p. 2. Emphasis mine.]
That section concludes by showing that "Only the gospel can supplement natural revelation" so as to disclose God's means of dealing with our enmity, and to make man once more a willing subject of the will of God. Natural revelation (or what is called here in this question 'Natural Law') can never achieve what only God does through his revealed word (in the Bible.) This answers the final question.
