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Why can't I destroy or break blocks?

I was playing Minecraft in single-player mode today when I turned on the "Open to LAN" option. I then changed my game mode to "creative" to fly back to my house, but when I turned my game mode back to "survival", I couldn't place anything down or break any blocks!

I've tried re-downloading Minecraft but that didn't help at all. It's just me in this world, so another user couldn't have changed any settings.

Why is this happening and how can I fix it?