If you are playing Single player, and not on a server: find where Curse is installed on your computer. (On my computer it is under Documents\Curse, but I have seen people say things like "C:\Curse") Under Curse, look for the "Minecraft\Instances" folder. Find the mod pack you are playing ("Minecraft\Instances\FTB Beyond"). Check that folder to see if there is a "backups" folder - if there is, you should have a bunch of zip files inside from all of the automatic world backups. If the "backups" folder is missing - or empty, you are out of luck.
Now check the "Minecraft\Instances\FTB Beyond" folder to make sure you have a "saves" folder - inside you should find a folder with your world name. If not, then something happened - you either manually removed the mod and re-installed it, or something happened on your computer. If the (world name) folder is there but does not show up when you launch FTB Beyond, then either that folder or the mod pack itself is corrupted.
To proceed past this point, you should have at least one zip file in the "backups folder". You may want to re-locate those zip files to your desktop for a few minutes: next you want to uninstall the mod pack and re-install it, to make sure you have the latest, uncorrupted version.
Finally, take one of those zip files (the most recently date-time) and unzip the (worldname) folder, placing it into your new "Minecraft\Instances\FTB Beyond\saves\(world name)" folder.
With any luck, you might have lost a few hours of work, but you should be back up and running!
