Has happened with me a hundred times, So i guess im the expert with this.
Search your computer, probably the c drive with the parameter ".torrent" (Without the quotes).
You'll find all the torrents you have used with utorrent till date. Copy those that were running (45GB data one).
Add them up to utorrent and they'll resume where they stopped. The biggest reason for this thing is improper shutdown.
I hope you'll find this useful.