First, check how you have Properties set for viewing Sessions in the Internal Database. In the IDB context, right-click on Sessions, and pick Properties, and make sure that these fields are set
Created In: <All Sessions>
From Type: <All Sessions>
I got caught by this once before I know what it was all about, drove me crazy
If the Properties are set correctly, then use the command line to find out if the session is there
omnidb -session -type backup -since [date] -until [date] Do not include teh [ ] marks
For '-since', I would use the day before the backup, and for until, I would use the day after the backup, so, for example
omnidb -session -type backup -since 2014/03/20 -until 2014/03/22
You can also use the 'omnirpt' command
omnirpt -report list_sessions -timeframe 2014/03/20 00:01 2014/03/22 23:59
and, if you know the name of the backup specification, you can filter the results to include only these backups
omnirpt -report list_sessions -datalist "[backup job name]" -timeframe 2014/03/20 00:01 2014/03/22 23:59
If neither of these finds your session, then, I would say that there is a fair chance that the media was ever-written, and the original backup is gone.
You can also examine the 'media.log' file (you didn't specify what OS your cell manager is on, so, I can't be more specific), find the daye the backup was made, each line should include the name of the Backup specification. This will identify what media was used for thsi backup. If you can find the media, it can be imported into the IDB, restoring your session information