How do I resume an interrupted download?
Many clients automatically "remember" where they left off, and also save the torrent to make the resumption of the file essentially seamless.
Azureus is such a client. It resumes automatically, is very configurable, has many plugins and can run on any operating system that can use java.
For those clients that do not auto resume
Open the .torrent again either by double clicking the torrent file on your hard drive (if you saved it), or clicking the link again on the web page where you found it. Direct the download to the same directory you started downloading it to originally BitTorrent will do the rest.