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