Ubuntu – “ALERT! /dev/mapper/ubuntu–vg-root does not exist “

When I boot up my server in Ubuntu 14.04 LTS Desktop .
I encounter this situation ,
after the screen tell you how to get into BIOS

Then

Gave up waiting for root device. Common problem:
  - Boot args (cat /proc/cmdline)
   - Check rootdelay= (did the system wait long enough?)
   - Check root= (did the system 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)_

after I typed exit , system could always start normally.

I have searched and found out that there are many similar problems like mine . But the answers I have tried cannot solve my situation. I'm going to describe my server environment and what I have done as detailed as possible , very appreciate for any help . It really made me annoyed because I didn't do anything special before last shutdown .

I have installed Ubuntu system with LVM mechanism.

sudo parted -l

I took this screenshot when the system just setup several weeks ago.

lsblk

I have two disks , sdb is for another volume group , logical volumes on that vg are for KVM . I think it's not the matter , just mention.

I've followed the hints to inspect :

- Boot args (cat /proc/cmdline)
 - Check rootdelay= (did the system wait long enough?) 
 - Check root= (did the system for the right device?)

what I've got

cat /proc/cmdline  
BOOT_IMAGE=/vmlinuz-4.4.0-45-generic.efi.signed root=/dev/mapper/ubuntu—vg-root ro rootdelay=30 quiet splash

As you can see , I've edited /etc/default/grub to add rootdelay=30, but it fixed nothing.

- Missing modules (cat /proc/modules; ls/dev)

Not really knowing what should I notice about this hint . When I list files in /dev/mapper , /dev/mapper/ubuntu–vg-swap_1 and /dev/mapper/ubuntu–vg-root are all there .

Other information that frequently asked to provide for similar problem :

cat /etc/fstab

blkid

I have entered into GRUB menu to boot with recovery mode , there are three pairs :

Ubuntu, with Linux 4.4.0-45-generic
Ubuntu, with Linux 4.4.0-45-generic(recovery mode)
Ubuntu, with Linux 4.4.0-42-generic
Ubuntu, with Linux 4.4.0-42-generic(recovery mode)
Ubuntu, with Linux 4.4.0-38-generic
Ubuntu, with Linux 4.4.0-38-generic(recovery mode)

But when I select any version in recovery mode to boot , just fail into BusyBox again . As I previously mentioned , after I type exit I could start system normally , but I don't think that I should do this every time , and it apparently my server have something wrong .

I really want to know what behavior or situation could cause this problem ,then I could prevent it .