See if you want to retrace your marksheet and all the documents, then the most probable way is that you need to contact your school or the Boards regional office itself in order to do the required official works for a new marksheet and other documents.

Now the general process is that you have to do a complaint on loosing of document to your local Police station and then you have to take the report to your school or the boards regional office and they will definitely guide you out regarding it. There are no other ways available.

Have a great eve!