If you have lost your world (meaning when you open the game your world is missing) this may be helpful. THIS IS FOR WINDOWS 10 EDITION ONLY
I have had this problem and have fixed it using this method (couldn't find anything to help so i figured it out then made this, so you won't have to figure it out yourself).
Just so you now my problem occurred because i had a power outage as my world was saving after i closed it. I don't know if it may work for you, but best to give it a shot, it's short and easy.
First off open Minecraft and make a new world (give the world the same settings as you old one, as the settings will change after the process). Save and close and save the world, then close Minecraft.
Then, open files explorer. go: This PC/Local Disk (C:)/Users/User/AppData/Local/Packages/Microsoft.MinecraftUWP_8wekyb3d8bbwe/LocalState/games/com.mojang/minecraftWorlds
If you have trouble finding the folder try this: https://help.ableton.com/hc/en-us/articles/209070509-How-to-access-hidden-folders
It is easier if you only had one world (I did) but there should be two (maybe more for you) folders in the minecraftWorlds folder that has a few random looking characters. As I said I only had one world so for me there was two folders, my old lost world and the new world I had just created. If you have more than two folders you may just have to do some trial and error, although they are in order of creation with oldest being at the top, newest at the bottom (the bottom-most folder is your new world).
Open you newest world folder (the one you made earlier) go to the db folder, delete everything in there (not the db folder itself. Go to your lost worlds folder, go to the db folder (for me there was another "lost" folder in db, that's fine). Copy everything in the db folder, including the "lost" folder. Go to your new world folder, go to the db folder, paste the contents from the old db folder into the new.
Open Minecraft, go to worlds (the world should still be called "My World", you can edit it later). Open the new world and you should find it has become your old world.
I hope this helps ! I have not seen anything else explaining this so I this helps some people, happy mining!