What exactly do you mean by "loss of detail"?
Itf it's simply underexposure on some parts (not all) of the image, Michael and Andrew have already answered - I'll just add that you have the same tool built into Adobe Camera Raw (ACR), and I think the Mac version of PSE4 can handle the
latest ACR plugin reasonably well.
If Fill Light cannot restore it, then it is probably a lost cause. NB if you're dealing with jpegs, it may probably be the case (but you can try, nevertheless - miracles do happen from time to time ;o).
You may have to add some noise reduction (chroma first, luminance in case of necessity), and sometimes to push the blacks slider a bit to the right (careful with that axe) to restore some contrast a bit.
From what I see in its page, the plugin you mention has essentially the same effect as Fill light, but as said Andrew you start with much less information than the RAW file.