Minecraft Windows 10 Troubleshooting Guide (version 1.16 and up)
Made by Gerek T. All sources that were used are credited.
----------------------------------

HOW TO RESTORE A CORRUPT MINECRAFT WORLD on Windows 10 Edition

Step 1: Identify the problem.

Step 2: Create a new world. Set the difficulty to peacefull.
-This creates new, uncorrupted files that we need to repair the old world.

Step 3: Open your worlds directory.
-To find the directory, go to your files app, go to quick access, and click the bar that says quick access, then copy this and paste it there: 
%LocalAppData%PackagesMicrosoft.MinecraftUWP_8wekyb3d8bbweLocalStategamescom.mojangminecraftWorlds

Step 4: Find your new world folder.
-After opening your minecraftworlds folder you should see files with names like "1g4AAHBsAQA=". These are your worlds. You can tell what world is what by opening the file and opening levelname.txt.

Step 5: Copy the new Level.dat and Level.dat_old files from your new world. Note that level.dat may display as only level in the world folder

Step 6: Go to the corrupt world and replace the two corrupt level.dat files with the two new ones.

Step 7: Go back to the folder with all of your Minecraft worlds, then right click on your world folder, and click "restore previous versions." A window should pop up, if so, click the "ok." button.

Go back to Minecraft, and your world should be there!
This will fix your save and make it playable once again. You won't spawn in some random spot like in java edition and you will keep all of your items.

These instructions were from https://www.reddit.com/r/Minecraft/comments/b4sa4p/guide_how_to_recover_a_lostcorrupt_world_on/
----------------------------------

HOW TO FIX THE "This world cannot be loaded" ERROR

NOTE: Back-up your world, in case it breaks. You can do that by clicking the edit icon next to the world (in the menu), click "game", scroll down, and click "export."

Step 1: Create a new world. For the world seed, use the seed from your world with the error.

Step 2: Enter the new world, and close the game WITHOUT SAVING (pressing the X button in the corner of the window)

Step 3: Go to the game's directory, by using this link in the files app: %LocalAppData%PackagesMicrosoft.MinecraftUWP_8wekyb3d8bbweLocalStategamescom.mojangminecraftWorlds

Step 4: Go to the new world's folder, then go to the "db" folder, and copy the "CURRENT" file.

Step 5: Go to the other world's folder, go to db, and paste the CURRENT file. A window should pop up, asking if you want to replace it, click YES.

Step 6: Exit the files app, and your world should be all fixed!

These instructions were from: https://www.youtube.com/watch?v=5qa9WyOQAhc