The 'fatal: authentication failed' error often stems from incorrect credentials or an expired access token, especially after GitHub deprecated basic password authentication. To resolve this, generate a personal access token (PAT) from *Settings > Developer Settings > Personal access tokens* on GitHub, granting appropriate scopes like 'repo' access. Once created, replace your password with the token in your Git configuration by running `git remote set-url origin https://username:[email protected]/username/repo.git`. If you use a GUI like GitHub Desktop, enter the PAT there when prompted. For network restrictions, check if your organization’s firewall or proxy is blocking GitHub. By updating these credentials and ensuring network access, you can restore successful Git operations.