Most exam portals let you re-download the admit card from the same link – log in with your registration number and date of birth again.
If the download link has closed, contact the exam helpdesk with your registration details and a copy of your application.
Keep a scanned copy of every admit card saved on your phone and email so you never depend on one printout.
For exams already over, the conducting body usually does not reissue the admit card – your scorecard or result PDF serves as proof.
Carry two printed copies to the exam centre in future, so losing one is never a problem.