Icon with a missing down arrow

What is the content of that folder? I guess you moved the folder off the toolbar and tried to retun it back, but TLB created the shortcut. So you can delete the shortcut and drag the folder to the toolbar by right mouse button. When you drop the folder TLB ask you what to do: Move Here, Copy Here, Create Menu Here

If you want to leave the content of the folder where it is now, select "Create Menu Here". If the folder contain just some shortcuts you've created in TLB select "Move" or "Copy".