Has It just disappeared from the Dock, or is it gone from your home folder?
If it's just disappeared from the Dock, open a new Finder window, click on your home folder (the house in the sidebar), and drag the Downloads folder back onto the Dock.
If the folder itself is gone:
Are you looking to recover what was in the folder, or just recreate a new Downloads folder?
When you're looking at your home folder in the Finder, click on the Time Machine icon in the menu bar and choose "enter Time Machine". Go back in time until the folder reappears, click it, and click "Restore" at the bottom of the screen. You can drag this restored folder back to the Dock.
Go into your home folder and choose "new folder" from the "File" menu. Name this new folder "Downloads" (upper-case "D"). It will be recognized as the regular Downloads folder. Now you can drag this folder to the Dock.