The 'Cannot lock ref' error in Git often happens when another process or a previous Git operation left a `.lock` file, effectively blocking the repository from making further changes. This can occur if a push, merge, or fetch operation was interrupted, leaving a stale lock on a specific ref (branch or tag). To resolve the issue, navigate to your repository’s .git directory and check for any .lock files under the refs or objects folder. For instance, you can use the command `rm -f .git/refs/heads/branchname.lock` to delete the lock file and release the ref. Be cautious, as improperly deleting lock files could cause issues in a multi-user environment. Once removed, try running your push or other Git operations again. Lock files are intended to prevent conflicts in concurrent Git processes, so removing them typically fixes the error.