Hi there,
the missing PSD layer-files should show up with Italic text names in your Project-Window.
Just double-click on one, and you get a fileselector prompting you to locate the file.
Now first locate the PSD file, and in the next dialog you get choose the correct Layer from the pop-up. AE should figure out what other layers it’s missing are in that file too, and reconnects them, so you only have to do this once for every psd-file, not for every layer.