It is acting like it is in survival mode. The inventory doesn't show all the materials. It looks like the survival inventory. I also can't fly, destroy blocks in one hit, you know the stuff you play creative mode for. Is there a way to fix this?

You are probably in Survival mode. There's a mod or something that adds an option to switch between Survival and Creative with the click of a button, similar to writing /gamemode *name* 1 in multiplayer

Yes, if you have single player cheats on you could make yourself be in creative mode. If not use a program called NBTedit. Edit the level.dat to change your self to creative mode.

It's always been in creative mode. Also, I don't use mods.

I'm trying to use single player cheats to change the default game mode back to Creative and it is not working. Am I missing something?

If I remember it's player ganemode 1/0 but if it's an old map you might not have the cheats on.