There could be several reasons why you are getting an all-black/grayscale image when loading a PNG file in MATLAB. Here are some possible explanations:

1. The image file may be corrupted or incomplete. Try opening the image in another image viewer to confirm if the issue is with the image file itself.

2. The PNG file may have a color mode that MATLAB does not support. For example, MATLAB does not support indexed PNG images that use a color table. In this case, you may need to convert the image to a different format that is supported by MATLAB.

3. The image file may have transparency information that is not properly handled by MATLAB. Try checking if the image has an alpha channel (transparency layer) and remove it to see if that resolves the issue.

4. The image file may have a color range that is outside the expected range for MATLAB. Try adjusting the color range using the imadjust function in MATLAB to see if that helps.

5. It's also possible that there is an issue with your code. Make sure you are using the correct file path and the appropriate image loading function (e.g., imread) in MATLAB.

In any case, try opening the image in a different image viewer to confirm that the issue is not with the image file itself. If the problem persists, you may need to experiment with different image formats or try adjusting the image properties to find a solution.