At least most of the default swatches are in the swatch libraries. Select the options in the top right of the swatches panel, and choose Open Swatch Library > Default Swatches > Basic RGB. There's other sets in there, choose the ones you think fit best.
Swatches are stored within the document. If a document doesn't have any swatches (to save disk space, for example), Illustrator won't display any. If you want access to your default swatches all the time, open them up as a separate panel and dock that somewhere in your workspace.