You can use umount -f -l /mnt/myfolder , and that will fix the problem.
-f – Force unmount (in case of an unreachable NFS system). (Requires kernel 2.1. ...
-l – Lazy unmount. Detach the filesystem from the filesystem hierarchy now, and cleanup all references to the filesystem as soon as it is not busy anymore.
参考:https://askubuntu.com/questions/292043/how-to-unmount-nfs-when-server-is-gone