Because they misunderstood the nature of the kingdom, they also misunderstood the nature of its King (both would be spiritual, yet they are still imagining a physical king and kingdom). The people either understood that Jesus spoke of His own death or they believed He was intending to leave the planet in some other way (perhaps like Elijah; e.g., II Kings 2:11). Yet, they could not reconcile either of these ideas with the notion of Him being the Messiah, for they believed that the Messiah would reign forever on Earth (cf. Dan. 7:13,14; Micah 4:7). Thus, they are having some serious doubts about Jesus. Although He had received the honors they gave to Him as the Messiah two days earlier during His triumphant entry into Jerusalem, they now, since He is speaking of dying or departing, have doubts that He really is their Savior.
Because of their preconceived ideas about the kingdom, they ask - "Who is this Son of Man?" Their loyalty to Jesus is wavering as it seems they are asking Him to distinguish Himself from the true Messiah. This question of doubt is a midway turning point between their shouts of "Hosanna!" (John 12:13) and "Crucify Him!" (19:14,15).
"A little while longer the light is with you. Walk while you have the light, lest darkness overtake you; he who walks in darkness does not know where he is going" - Jesus does not answer their question for He had already done more than enough to prove His claim of Messiahship. Had He attempted to explain to their carnal minds how He could be "lifted up" and yet remain would have only resulted in contempt. Instead, He speaks this warning to them. Light, when used metaphorically in the Scriptures, is a synonym for truth; darkness represents error and wickedness. "While you have the light, believe in the light, that you may become sons of light" - For the moment they had "the truth" with them (i.e., Jesus; John 14:6). He exhorts them to cling to the truth to prevent them from wandering blindly in the darkness.
"These things Jesus spoke, and departed, and was hidden from them" - At this point Jesus leaves the crowd, though their faith in Him had already departed. He would never personally teach them again. They had just rejected the greatest teacher that ever lived!
It is likely that Jesus went to Bethany after this encounter. The multitudes would see Him no more until He was a prisoner in the hands of His enemies.