If you find your libraries don't contain components or symbols that are used in some of your designs, those items can be 'recovered' from the design files to rebuild your libraries.
Symbols should be imported before the matching components for V11 and earlier, as the symbols must already be present in the correct library for the links to be formed correctly. With V12 and later it's desirable, but not critical. The procedure for both types of symbol is the same, so only schematic symbol import will be described.


In Easy-PC for Windows, go to [File], [Libraries], and click on [New Lib.]. Create a new working library plus new libraries matching the names of any lost libraries. Next, select the working library then click on [Add File]. Browse for the the schematic files containing lost symbols, and click on [Open]. All schematic symbols found in the file will be added to the working library. Do this for each file. In most cases you can overwrite any previous copies of symbols already recovered, but obviously if there are specific differences between identically named symbols, you will need to manage the conflict**.
When all the symbols have been recovered, including duplicates and the ones you didn't really want, use the [Copy To] button to copy the relevant symbols to the appropriate new library. When you've finished, either select all the contents of the working library, and delete them, or use Windows to delete the library file. Either way will prevent problems with duplicate symbol names.
The procedure for component libraries is similar, but with one important difference. It's not enough to just add a schematic or a pcb file. Unless the component is schematic or pcb only, you must add both schematic and pcb files containing the component to build the full library model. If the component used multiple packages, you must add pcb designs for each package as well, or the neglected packages will be lost.


With recent versions there is an alternative method for adding to libraries.  Select any components which need to be added to your libraries.  This can be done using any selection method, including frame selection as items other than components will be ignored. Now right click and use Add To Library.  This brings up a dialogue showing all the components and symbols found, also allowing you to choose to which libraries they are to be saved.  As before, components need both schematic and pcb elements, including multiple packages where relevant.

**Note - From V12, components only keep a note of the symbol name, not the library, so it IS important to avoid duplicate symbol names. Because components search for symbols, only the first occurrence of any duplicate symbols will ever be used.