This question already has an answer here:
So I was building a farm house and as I built it, I was in creative mode. After it was built, I decided that I wanted to be in survival mode and just mess around and possibly build my farm house even bigger. Welll..... I can not get out of this weird mode. I do the correct /gamemode and I see my hearts and food bar show up but I'm not able to break blocks as if I'm in survival, they just completely disappear after i break them! Like if I was in creative mode, you know how you break a block and poof its gone, well its like that. I see the little block after i break it but its only for 1 sec. I cant fly or anything when I'm like this just to be clear, I also kill everything with one hit and get no drops and only one experience rolls my way.
I've tried restarting everything and also using a different version than my original 1.8.9 minecraft. Its also on my pc... And another thing, everything works fine in my other saved worlds, Just this particular one, that is messed up.