add library gone missing?
Basically what can happen when you create a new project is that Eagle will initialise a new project but with none of the libraries enabled. Once you re-enable them, it will store which are being used in the project file and remember for next time.

There are two ways of enabling libraries:


All at once

You can enable all the libraries all at once by going to the Eagle Control Panel, finding where it says "Libraries" and right clicking on it, then selecting "Use All"


One at a time

You can enable libraries one at a time in the control panel. In the picture above, notice the little green circles next to each library. If the library is not being used, the circle is grey. If you click on it, it will go green and the library will be enabled (click again and it will be disabled).

It is also possible to do it one at a time in the in both the Schematic editor and the Layout editor. In the editor, select where it says "Libraries" on the menu bar, and click "Use...". This will bring up a window where you can select one or more libraries to use.