They sent a letter saying you "MAY" be eligible for the EIC (not a certainty) and asked you to fill in the form 15111 and return it so they can consider it. Now all you can do is wait until someone finally gets back to your return in the pile up of millions of returns ... this may take months to resolve and there is nothing you can do to speed up the process.
Review the notice they sent originally ... there should be a phone number you can call and/or a fax number you can try to resend the form 15111 although both options will probably not work. Did they ever release your original refund ? Did you check your IRS return/account transcript for a clue as to what is happening? https://www.irs.gov/individuals/get-transcript