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Does the port number listed in your preferences correspond to the one you forwarded in your router? Who knows, it may have changed.
 
Try deleting the torrent from transmission's download queue. Then drag the .torrent file for the download back into transmission. Transmission will scan the directory it downloads into and show that it has already downloaded some of the content for the torrent. It will then resume the download from where it left off.

Hopefully you can still find the .torrent file for your dl.
 
If you have your router port forwarded you should unselect "Automatically map port." Automatically mapping the port while also port forwarding the port can create issues.

I would suggest that you remove the port forwarding and see if automatically mapping the port works. When you automatically map the port you do not need to forward the port; instead the app sends a signal to your router telling the router that the app needs that port open. If your router supports NAT-PMP or UPnP the router will open that port while the app is open.

Automatically mapping the port is more secure as the port is only open when the app is open. I find that support for this feature is not great depending on the router you have. You may have to forward the port manually as the mapping may not work. If you have a Mac and an Airport Extreme it works flawlessly.
 
Try completely deleting Transmission, including the "home/library/application support/transmission" folder and "home/library/preferences/org.m0k.transmission.plist" file.

Then reinstall transmission. You will have to reconfigure all of your preferences but if you are having problems this may be your only solution. I would also change the port and port forward the new port just to eliminate any possible sources of your issue.

Also, when you port forward a port, it works much better when you make seperate port forwardings for tcp and udp.
 
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