application folder disappeared from dock. how do I get it back?

The add the full Applications folder to the Dock:

1) Make sure the Finder is active

2) press Command-Shift-A to open a window showing the Applications folder

3) press Command-Up Arrow to move one level up

4) drag the Applications folder to the Dock, to the right side of the separator (assuming the Dock is at the bottom).

If an app disappeared from the Dock, open the applications folder and drag the app into the Dock. If you are talking about Launchpad that is also in the applications folder.