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 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.