I tried to comment out some code for testing using <!-- Code to commented out while testing -->. When I type <!-- all the code after that disappears. I'm sure this is something that I'm doing or not doing, but this has never happened before. I'm completely puzzled

By 'disappears' you mean that it is grayed out. That is normal. Go on down to where you want to end the commented section and put in the --> and the code after that will be there again.