According to "man umount" you should be able to use umount -f or umount -l to force the unmount (although -f is for NFS commands, so you'd want to try the -l on a local device; check your man pages to see that you have it supported on your kernel version)
What kind of failure did you have? You could try to see what processes are holding it open with "lsof" then kill those processes if that's what is causing the hold on the filesystem.
According to "man umount" you should be able to use umount -f or umount -l to force the unmount (although -f is for NFS commands, so you'd want to try the -l on a local device; check your man pages to see that you have it supported on your kernel version)
What kind of failure did you have? You could try to see what processes are holding it open with "lsof" then kill those processes if that's what is causing the hold on the filesystem.