Welcome to LQ!!
First, please tell us more about your system--eg what version (distro) are you using?
Also, what kind of package are you trying to install? eg .rpm, .deb, .tar.gz, etc. (I assume it is something that was not available with the package manager.)
You can try looking in "ps -e" for the package manager still running. Then kill it with "kill -9 <PID>"