So, what is this error that comes when i started to boot ubuntu 14.04. How to solve this?

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/disk/by-uuid/4e2e4952-f2b0-4458-afe4-22f8cbe53ac1 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.    

I had this problem few weeks ago. In the busybox, can you mount your disk ? if 'yes', your disk is still seen by the system.

execute the following:

sudo blkid

it will display the uuid's of your disks. check that they are ok with the grub file.

Try rebooting on an older kernel via grub, it should boot. Then, in a shell, execute :

sudo update-grub

and reboot again on the newer kernel, it should be ok.

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