Okay, so, I recently started a new world, and I have just made a nether portal. When I entered, I could not break, nor place blocks. I was attempting to get up a hill, and could not place or break a block to get up it. I attempted to fix it by claiming the nearby chunks, thinking it was an error with ownership of the chunks, and I still cannot mine any blocks. It does not even show the outline for selecting blocks.

UPDATE: Whoopsie daisy! Apparently, while changing gamemodes through F3+F4, I had changed to Adventure Mode instead of Survival! Oopsies!