Today I get a problem when I can’t highlight the text for editting using Illustrator CS4, but when I try create a new document, it works normally.
After googling, I find that this document is set to “Hide Edges” when I hit Ctrl + H. So, I hit them again and the problem solved.
You can also see this function by click on the menu: VIEWS/Hide Edges (Ctrl H).
Hope this to be useful for you.