Send polite letter to Sears President or CEO requesting explanation and correction of problem (get fax # and and name and address from Yahoo biz, fax with confirmation sheet to show sending). If that doesn't work (keeping mind that Sears is not great on customer service) complain in writing to your State Attorney General's consumer protection office. Many states have laws prohibiting expiration of gift certificates.

By the way, don't shop at Sears. For example, I like Kenmore appliances, but recent shopping for a new dryer found the pricing uncompetitive and the service not too good. Sears is a great old name, but it is now in the hands of looters.