Why didn't the Egyptians believe Moshe when he warned them that each plague was going to come? Why didn't they bring their cattle inside before the plagues of Barad (hail)? The Kotzker Rebbe explains that the Egyptians lived in complete "darkness" - they refused to believe that God was more powerful than they were. That's why God brought the plague of Darkness - it was middah kineged middah (measure for measure).
We, too, must ensure that we do not live in darkness. Every day we should ask ourselves: What is my purpose in the world? Why am I here? If we do not live with purpose, then we are living in darkness.