When you launch Minecraft with a mod built on top of 
Alexandria Lib, the game expects Alexandria Lib to be active in the background at the exact same time. If it is missing or misplaced, your mod loader will pause startup before reaching the title screen and show a missing dependency message.
Checking Your Game Folders
If your game stops on a warning screen asking for Alexandria Lib even though you already have the file on your computer, check how it is stored:
1. Click the button on the error screen to open your mods folder, or navigate there manually from your Minecraft directory.
2. Verify the Alexandria Lib file is directly inside mods and not accidentally dropped into resourcepacks, config, or a subfolder.
3. Check the end of the file name to make sure your launcher did not append .disabled to it.
Verifying Loader Compatibility
Once the .jar file sits cleanly inside the mods folder alongside the mods that depend on it, close the game window and launch Minecraft again. With the shared base present, the dependent mods can access their core tools and finish loading to the main menu.