Hey

In case your CBSE admit card gets lost then you should immediately contact your school authorities and ask them to issue you a new admit card. In most cases the school has a soft copy of all students for avoiding any circumstances and they will provide you the admit card.

For your Roll number , you may also ask that to the school, they contain all records or you can talk to your friend who has a roll number closer to your roll number because only the last few digits differ and that is also in a numeric order, so that can be figured out.
For further doubts, ask us. All the best

Hii!
Don't worry!
There's a provision in CBSE where it allows you to download the duplicate admit card. So, for that, you need to visit the official website, and enter school affilation code as user name and school code as password. You may get your duplicate admit card and get the roll number from there.
Else, you can email the cbse helpdesk and ask for the solution.
Hope this helps.
Good luck!