World won’t open

The Minecraft app will not open any of my worlds. The loading bar will move across the screen but then just sit when it gets full and the facts will just keep changing every minute or so. I did create a new world and that was able to start up and when i saved it and went back to the home screen, i was able to get back into it later. However, if this is the solution for the other worlds, i can’t get in them to try and save the world