Department Of Insurance. In your state this case they will have to send you a reply to your written complaint within short period.
Call the department of insurance and do that.
Before you do that call and speak with a supervisor or manager they will talk to you and explain the situation. usually they will show you where in your policy it states that the loss is not covered or what was the problem.
You need to be more specific.
Is this your insurance company?
Do you have a claim with Nationwide, but not insured with them?
Is this auto/homeowners insurance?
Is this with the agent for Nationwide?
Did they deny a claim?
You can always file a complaint with the state insurance department where you live. Most websites have complaints you can file online.
good luck
Just write it down, and mail it in. But that's not going to do any good.
The person to complain to, that actually might HELP you, is your state insurance commissioner.