我刚刚在 USB 2.0 驱动器上安装了 14.04.1。启动时，系统提示我输入加密密钥，我输入了。但之后，我得到了以下信息：
Gave up waiting for root device. Common problems:
- Boot args (cat /proc/cmdline)
- Check rootdelay= (did the system wait long enough?)
- Check root= (did the system wait for the right device?)
- Missing modules (cat /proc/modules; ls /dev)
ALERT! /dev/mapper/ubuntu--vg-root does not exist. Dropping to a shell!
BusyBox v1.21.1 (Ubuntu 1:1.21.0-1ubuntu1) built-in shell (ash)
Enter 'help' for a list of built-in commands.
(initramfs) _
我以前从未见过这种情况，而且我不知道该怎么办:'(