If you are using Windows or Linux as Cell Manager, you can use real disaster recovery feature that is already implemented in Data Protector.
If you do not want to use disaster recovery feature for purpose of your question, you can find info on your media used for IDB backup in media.log and obrindex.dat files. When disaster occurs, reinstall OS and DP. Configure your backup device, and import media used for latest IDB backup (found in media.log). When media is imported, find session ID of your IDB backup.
Next step is to restore your IDB. I usually use omniofflr command:
omniofflr -idb -session <session_ID> -autorecover -force
I repeat, much better option is to use Enhanced Automated Disaster Recovery feature of Data Protector for purposes of Cell Manager recovery.