Immediately contact Social Security (800-772-1213 M-F 7am to 7pm) to see if they have a record of you enrolling on Part A. If they find out that there is no record that you enrolled onto Part A, you can sign up for it when you apply for Part B. They will make an appointment for you to come to the closest Social Security Office and may ask you to bring a copy of your birth certificate along with 2 forms from Social Security (one to be completed by the employer; the other by you) to be eligible for a special election onto Part B due to loss of employer coverage.
Once you are enrolled onto Part A (hospital benefit) and Part B (outpatient services benefit) of Medicare, you will be able to enroll onto a Medicare supplement and stand-alone drug plan /or/ a Medicare Advantage HMO plan.
Make sure you contact your Human Resources department to obtain verification that your employer drug plan coverage since turning 65 is comparable to a Medicare drug benefit. This verification may be requested of you by your new drug plan carrier should they believe you did not have comparable drug coverage and are eligible for a penalty.