I can't destroy blocks in Minecraft Creative Mode (singleplayer)

Solution 1:

First things first.

F1 makes your Minecraft become into like a Camera Mode. It removes your inventory and hand that you can see again if you press F1 again.

Next.

If you cannot destroy blocks, either you are holding a sword, or you are in Adventure Mode.

You can change back by executing the command (pre 1.13):

/gamemode 1

Edit: The new command post 1.13 is:

/gamemode creative