To get new certificate,

First of all you need to make an FIR from the area you have lost the certificate.
Send an application attaching the FIR copy to CBSE Delhi.

Then go to cbse regional office for issuing new marksheet or go to cbse head quater pritvihar delhi., There you will get to know about further procedure. They'll tell you what you have to do next.
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You can get your marksheet within 2 month or within 15 days.