See this IRS website for information on the 4883C letter you received - https://www.irs.gov/individuals/understanding-your-4883c-letter
Did you receive an IRS notice in the mail? If so, what is the notice number which should be located in the upper right of the first page usually preceded by either a CP or LTR?
See this IRS website for information on the 4883C letter you received - https://www.irs.gov/individuals/understanding-your-4883c-letter