Welcome to the forum!
All I can
see in the file is the blue rectangle. But there are other things in the file. I'm not sure why they aren't visible. Possibly because they are masked with clones, although I'm not sure if that's why. Oh, well the clones have Unset fill and stroke. And I'm guessing, it's the unset fill which makes them not visible.
You could try:
1 - Release the mask
2 - Change the fill and stroke of the masking path
3 - Re-set the mask
However, I'm not sure why a mask is being used in this case. The reason to use a mask is so that you can vary the visiblity of the objects under the mask. If that's what you want to do, you definitely should change the color and/or opacity of the masking path. Otherwise, probably clipping is more appropriate. In my test, using clipping made the objects visible, without changing the fill/stroke color.
Hint #1 - When you select something with the Selection tool, you can look at the status bar to find out what kind of object it is. That can help you figure out what to do next.
Hint #2 - You can use the Tab key to select one object at a time. It even selects invisible objects.
Let me know if you need further hints
