If you're using EPiServer Search for your site, your content needs to be indexed regularly to be searchable. Normally the index is rebuilt on site startup, and on various events like publishing/moving/deleting pages and files. If you need to manually force a full content re-indexing, there is a tool for that.
Browse to http://yoursite/EPiServer/CMS/Admin/IndexContent.aspx
On that screen you can click the "Start Indexing" button to rebuild the index.
There is an option, checked by default, that will remove old data first and then create the index.