Two things to try:
Drag your Downloads folder far enough off the dock so you see the little poof of smoke and drop it so it vanishes. Then, open a Finder window, and right click the Downloads entry on the left side of the side bar, and select Add to Dock. Test the newly added icon. If you do not see Downloads entry in your sidebar, click the Finder menu (by the apple) >> Preferences >> Sidebar and put a checkmark beside it.
If that doesn't work...
Open a Finder window or click your desktop in a blank spot so it says Finder by your apple. Then, click Go at the top of your screen, hold down Option, click Library on the menu. Locate your Preferences folder and double click it. Drag com.apple.dock.plist to your desktop. Reboot. Test. If that works, trash the file you just put on your desktop, otherwise, put it back where it came from.
Note, with the second option, you'll have to redo non-default dock icons.