yikes! It's me again----help!

I have done this several times before but right now I seem to have a deteriorating brain and apparently can't remember what I did before.  Here's the situation.  I bring one image up on the screen. Then I open another image. I put the second image on top of the first. Now, I lower the opacity of the top layer so I can see underneigth it because I want to erase part of the top layer and I need to see just where I want to erase. Like I said, I've done this before. When I'm done I bring the top layer back to 100% opacity and everything is fine---except tonight. Tonight I lowered the opacity and when I chose the eraser tool and clicked to start eraseing a box appears telling me the layer must first be simplified, do I want to continue. If I click cancel, I can't go any further. If I click "ok", the layer is simplified BUT THE OPACITY SLIDER GOES BACK TO 100% without actually making the image 100%. The image is opacity reduced but the slider has it at 100%.  What am I doing wrong?