The time to build your professional network is before you need it. Once you need help, it may be too late to find allies who are ready, willing and able to provide it. Among the tools that can help you grow your network is LinkedIn. com — a free, searchable database of professionals in virtually every field. Use LinkedIn to connect with current and former contacts, the contacts of your contacts and so on — just as you may use in-person opportunities to generate such connections. Also, use LinkedIn to initiate contact with strangers with whom you share common ground;…