If your instances have disappeared from the app, most likely you'll be able to get them back! If you accidentally deleted them, or checked "Delete Application Data" when uninstalling, your instances can not be recovered.
Check to see if you still have the instance files
Go to your app directory. You can find the path at the top of the app's settings.
If you have folders, like above, skip to the Recovering instances section.
If you previously set a different custom app directory, check in that location.
If you have recently upgraded from a really old version, like 0.7.1. You can also check in in the
%appdata%/com.modrinth.theseusfolder.
Recovering instances
Move all of your instance folders to another location for now.
For example. a folder on your Desktop
For every instance:
Was the instance paired to a modpack, and you want it to still be paired?
Yes: Download the modpack from within the app, making sure to select the same modpack version. If your modpack was up to date, this will be the latest.
No: Create a new instance with the same name, loader, and Minecraft version.
Replace the newly created instance folder with the original.
