What was king Nebuchadnezzar's dream?

King Nebuchadnezzar dreamed about a magnificent statue. The head was made of gold, the breast and the arms of silver, the belly and thighs of bronze, the legs of iron, and the feet were partially iron and partially clay. Suddenly, a rock, that was cut without hands, struck the feet and the whole statue crumbled apart until it was nothing.

Additional information:Daniel 2:39-40 "And after thee shall arise another kingdom inferior to thee, and another kingdom of brass, which shall rule over all the earth. And the fourth kingdom shall be strong as iron: forasmuch as iron breaketh in pieces and subdueth all things: and as iron that breaketh all these, shall it break in pieces and bruise." The four world empires here predicted as part of Nebuchadnezzar's dream are generally understood to have been the Babylonian (head of pure gold), Persian (breast and arms of silver), Greek (belly and thighs of brass), and Roman empires (legs of iron and feet and toes of partially iron and partially baked clay). From the days of Daniel to the coming of Christ, the world was ruled by these four empires, exactly as Daniel had predicted. With regard to the four empires, critics who assign a Maccabean date to the book of Daniel, in order to explain it as referring to past events rather than being a correct prediction of the future, find it necessary to place all four empires before the date of composition, that is, before the Maccabean revolt. However, nothing happened in the Maccabean period that answers to the rock that was "cut out..., without hands" (Daniel 2:44-45). These verses allude to a fifth kingdom, an eternal kingdom of God that will never be destroyed, that will not be left to another people, that will bring an end to all other kingdoms. This prophecy of the four kingdoms is further explained under different images in Daniel chapter 7 (the four beasts), chapter 8 (the ram and the goat), chapter 9 (the 70 weeks), and chapter 11 (the struggles between the kings of the north and the kings of the south).