I take it your trying to open the file in a lowe version of Photoshop, that could be a problem i guess although normally what your describing is a "flattened" PSD. It basically lets you flatten a PSD into a single layer without affecting the other layers. I don't think it's possible to unflatten it though without the original.
Flattend PSD's can be made with any version of Photoshop it's just an option you have. your using a different version of photoshop so it's probably a compatability issue.
Your trying to open it in PS7 so the layers might combine because of features that have been used in CS2 that are not available in PS7. If thats the case then no, i don't think there is anything you can do about.
Or you could ask the person who has PS CS2 to save it to be compatible with your version. But I doubt this is the problem. I believe that the person who gave you the PSD, gave it already flattened. I don't think there's a so big difference between PS 7 and CS2 about how they handle layers...
all psd files have a hidden flattened layer in it that ps uses for thumbnail, cache, etc.., chances are the cs2 file wasn't saved with maximum compatibility ( which is what i think is needed to display in an older version of ps ) therefor it only shows the flattened layer and cant read the others...i would think thats the most sensible answer..