Well, so long as you were not arrested or charged with any crime, then a purely civil restraining order should not impact your N-400. In fact, there are no questions about civil cases or judgments on the form and no parts of the good moral character requirements that say anything about civil cases. As far as the FBI checks go, I don't even think those searches reveal civil cases (but I'm not 100% sure about that). So I don't think you need to either tell the USCIS about the civil order and I don't think they will discover the civil order in your background check. However, if you were arrested but the charges were dropped, then you will still need to put this on the N-400, which might lead the USCIS to discover the civil restraining order. But even if the USCIS does find out about a civil restraining order I would be surprised if your N-400 was denied, if this was the only moral character issue.