When starting a new file, I placed a rectangle object and it had a color and fill. I accidentally hit a button or two, and then the color and fill went away and the object became invisible/transparent; I could not undo the action. I have looked all over and read and tried many of the proposed solutions people have given for this, and was unable to figure it out. In my .svg, the object's opacity is 100%, the fill's alpha (opacity) is 100, both the layer and object do not have the "hide" eye button pushed, and the object it is not below any other object or layer.

I would upload my .svg file here but I do not see where I can do that. Thank you.

 Can you check "View->Display mode->Normal is on?

Have you looked into the .svg itself, i.e. with Inkscape's XML-Editor or an external Text-Editor where it might be easier to search "opacity=", "display:none" and "display=none" (which might sit in a higher-level group to be inherited by the object)?