'How do I get document and download stacks back on my dock?' - ...
To place the 'Documents' and 'Downloads' folders, back onto the 'Dock':
01. Navigate to the current users' 'Home' folder.
02. Drag the 'Documents' folder onto the 'Dock'.
03. Drag the 'Downloads' folder onto the 'Dock'.
If the 'Dock' is along the left or right side of the screen - you will drag the 'Documents' and 'Downloads' folders to the lower portion of the 'Dock' (above the Trash can).
If the 'Dock' is along the bottom of the screen - you will drag the 'Documents' and 'Downloads' folders to the right side portion of the 'Dock' (to the left of the Trash can).