Most modern browsers support resumed download as long as you do not remove the file from its download list as long as it detects this is possible (i.e. the file is as it left it and hasn't changed on the server since the last request) so you could just retry the download.
Some servers don't support partial downloads (though this is rare to find these days) and your browser may not retry if you have restarted the browser since the dowload fail (Firefox 3 added cross-session restarting to that browser line, I don't know about others), so you might not have success with just retrying so I suggest you take a copy of the parts you currently have so you can try something else afterwards if this fails.
You could also try
wget's -c option. From the man page:
Continue getting a partially-downloaded file. This is useful when you want to finish up a download started by a previous instance of Wget, or by another program. For instance:
wget -c ftp://sunsite.doc.ic.ac.uk/ls-lR.Z
If there is a file named ls-lR.Z in the current directory, Wget will assume that it is the first portion of the remote file, and will ask the server to continue the retrieval from an offset equal to the length of the local file.
which seems to be exactly what you want to do. This will not work if the original download client pre-allocated space for the file as it will already look to be full length - if this is the case you will need to truncate it to the length that you know is correct first. Also, it will not work if the original downloading app uses multiple connections per download as there will be gaps within the portion that you have and wget will not detect these.
(as for download managers: I use the DownThemAll Firefox extension which may be no use to you as you presumably prefer Chrome, I know someone who uses multiget which is in either the universe or multiverse repository of Ubuntu though I've not used it myself)