Can't open Worlds

I can't open worlds.

If I want to poen a world it just sais opening world and when it's arrived at 100% it crashes and puts me back in the world list.

Even if I create a new one I can't play in it.

I don't know why but servers don't seem to have any problem.

I have a world for 3 years, and I don't want to lose it.

thanks DarkDado14