As DMarkM1 suggested, the best course of action, in this case, is to contact the IRS directly. They are the ones holding your refund.
If 60+ days haven't elapsed since the IRS received the additional information you sent to them (Notice 12C is a request for additional information), then I suggest you wait till at least 60 days pass, and then call.
If after you contact the IRS they are not able to help, then use the link that DMarkM1 provided and contact your local Tax Advocacy.
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