You can use rsync to resume a broken scp transfer.

rsync -P -rsh=ssh file.tar remote:file.tar
rsync -P -rsh=ssh remote:file.tar file.tar

--partial                keep partially transferred files
--progress               show progress during transfer
-P                       same as --partial --progress

Background

Assuming you want to push a big file to a remote server and for some reason the connection broke. It would be a pity to loose all of the already transferred bytes and you might wonder, if there is not a way to resume a failed scp transfer. While scp cannot do that, rsync can take on a failed transfer.

Using one of the methods above allows you to resume a failed transfer.