If you're on Windows 7, this may be because on the 22nd of February 2018, GitHub disabled support for connections using a version of TLS older than 1.2, and Windows 7 doesn't support TLS 1.2 by default. Windows 7 users must ensure they have the latest Windows updates, and that they have enabled TLS 1.2 support.
-If LOOT still can't update its masterlist, it's likely due to connectivity issues that are beyond our control. You can update it manually by downloading this file and placing it in %LOCALAPPDATA%\LOOT\prelude\. Then, download the masterlist for your game from this location and place it in %LOCALAPPDATA%\LOOT\games\<game folder>\. Depending on your game, replace game within the URL with morrowind, oblivion, skyrim, skyrimse, skyrimvr, fallout3, falloutnv, fallout4, fallout4vr or enderal, and <game folder> within the folder path with Morrowind, Oblivion, Skyrim, Skyrim Special Edition, Skyrim VR, Fallout3, FalloutNV, Fallout4, Fallout4VR, Nehrim, Enderal or Enderal Special Edition. Note that Oblivion and Nehrim share the oblivion masterlist and Enderal and Enderal Special Edition share the enderal masterlist. Also note that you'll want to disable the "Update masterlist before sorting" option in LOOT's settings until your connectivity issues have been resolved.
If LOOT still can't update its masterlist, it's likely due to connectivity issues that are beyond our control. You can update it manually by downloading this file, selecting "Open LOOT Data Folder" within LOOT's File menu and placing prelude.yaml in the prelude folder. Then, download the masterlist for your game from this location and place it in games\<game folder>\. Depending on your game, replace game within the URL with morrowind, oblivion, skyrim, skyrimse, skyrimvr, fallout3, falloutnv, fallout4, fallout4vr or enderal, and <game folder> within the folder path with Morrowind, Oblivion, Skyrim, Skyrim Special Edition, Skyrim VR, Fallout3, FalloutNV, Fallout4, Fallout4VR, Nehrim, Enderal or Enderal Special Edition. Note that Oblivion and Nehrim share the oblivion masterlist and Enderal and Enderal Special Edition share the enderal masterlist. Also note that you'll want to disable the "Update masterlist before sorting" option in LOOT's settings until your connectivity issues have been resolved.
Steam removes the game's Registry entry when the game is updated or its cache is verified. Run the game's launcher to add the Registry entry back again. LOOT will then be able to detect the game.
Another cause may be that the game is installed in a location where you do not have read permissions. In this case, LOOT cannot read your plugins and treats the game as undetected. Try checking the permissions of the game's path and ensuring you have read access.
-You can also enter your game's install path manually in LOOT's settings menu. In the menu, select your game's settings entry on the left-hand side (if it's not there, delete LOOT's settings found at %LOCALAPPDATA%\LOOT\settings.toml to restore them to default or make a new entry for your game) and enter your install path in the "Install Path" field.
You can also enter your game's install path manually in LOOT's settings menu. In the menu, select your game's settings profile in the left-hand side (if it's not there, select "Open LOOT Data Folder" within LOOT's File menu and delete settings.toml to restore your settings to default or make a new profile for your game) and enter your install path in the "Install Path" field.
plugins.txt and loadorder.txt. Won't this break something?