A couple of other things that I've thought of.
1) Do not call the number on the letter. Especially if it's not a 800-829 number. Granted, not all IRS phone numbers are 800-829 numbers, but a lot are.
2) Your best bet is calling 800-829-1040. But, as I previously mentioned, that number is probably swamped right now and will be up until April 15. I'm hoping you just got the letter in the past few days. If so, wait until after April 15 to call. The IRS always allows at least 30 days to respond to correspondence, so you should be OK.
3) If you still can't get help, refile your 2012 tax return. Make sure you attach all forms and schedules, and most importantly, sign your return in pen and ink. Mail it to the service center where you normally file your tax return (check irs.gov if you don't know). If you never filed your tax return, you'll receive your refund. But if you previously filed and received a refund, you'll get a notice from the IRS stating there is already a 2012 tax return in its database with your name and SSN. Call the number provided in that notice (chances are, it could be an idenity theft unit), and tell them your story. They'll be interested in another idenity theft scam.
Good luck.

The IRS may contact some people this time of year if they had a lot of income and might not have filed. You may have received your state refund and your federal return might not have been processed. Returns do get lost, and people do forget to send returns in. Don't worry if you legitimately forgot to file, especially if you're due a refund. As long as you file your 2012 refund return by April 15, 2016, you'll receive you refund, no questions asked. And with interest.
