If you start downloading a torrent which was previously active, the client will first try to resume downloaded data from the disk. The resume progress is shown in the resume dialog, depicted below:

+--------------------------------------- Download ---------------------------------------+
|                                                                                        |
| bittorrent:http://www.legaltorrents.com/bit/blue-a-short-film.torrent                  |
|                                                                                        |
| [||||||||||||||||||||||||||               26%                                        ] |
|                                                                                        |
| Resuming                                                                               |
|                                                                                        |
| [ Background ] [ Background with notify ] [ Info ] [ Abort ] [ Abort and delete file ] |
+----------------------------------------------------------------------------------------+

Wait until the resume completes. It will automatically launch the download dialog.