You can't take a T-Log backup if your database is in Simple recovery mode. So, basically you can't do point in time restores. In other words, The earliest point you can go back(restore) is to the time you've your latest Full backup.
However, You could consider differential backups to minimize the data loss under SIMPLE recovery mode. If you want point in time recovery, you should be using FULL Recovery model and take T-Log backups frequently(with no log backups under FULL Recovery model, your T-Log will simple grow and grow......)
See this: http://technet.microsoft.com/en-us/library/ms191164(v=SQL.105).aspx
Hope this helps.