You should check with your local post office if they sent you the original certificate via Indian post. The original mark sheets/certificates are sent to the candidate at their registered address, but they may or may not reach the student. You have the following options available to you.
1. Visit a regional study center and inquire if they have your original certificate.
If they have it, they will issue you a certificate after the necessary formalities.
2. If not, go to IGNOU headquarters in Delhi (allow a couple of days before going, but staff members at Delhi HO are very helpful) and they will provide you with a duplicate copy of your mark sheet/certificate.
3. When you visit the IGNOU headquarters in Delhi, try to obtain the dispatch number as well as other pertinent information and establish contact with the postal department.
Otherwise, with the above information, file an RTI regarding the status of delivery of your consignment as per the information received from IGNOU. Because you will face difficulties if you do not present original certificates at the time of admission/interview, make every effort to obtain the originals.