Need to open a recent project to see how you achieved an effect? You don't need to close your current project. Go to File>Import or press Cmd+I (Ctrl+I) and import that project.
This works best if you've been organized and put all your source files in the same folder as your project file. If After Effects can't find a file, it substitutes a placeholder.
To re-link or replace a reference:
Select the placeholder or footage file in the Project window, and choose File>Replace Footage>File or double-click on the placeholder.
In the Replace Footage File dialog box, locate and select the footage file you want to use.