recovering the old user

Root can do the file renamed in root recovery console (accessible from GRUB). Alternatively, boot to the Ubuntu install media, mount your internal partition and rename the file as needed.

In comment #24 in https://answers.launchpad.net/ubuntu/+question/688850 you wrote

"I found some weird messages coming and repeating themselves
Here are few lines
Print_req_error : I/o error , dev sda
ata1.00 : status :EDRDY ERR3
ata1.00: error :EUNC3
ata1.00:irq_stat 0x400000008
ata1.00:failed command :read FPDMA Quoted"

In my opinion these messages indicate severe problems with your hard disk, and I recommended that you immediately create backup copies on another medium, if you have data on that disk that you want to preserve.

What happens if you boot your system from an Ubuntu installer (either from a bootable USB stick or from a DVD) in the "Try Ubuntu without installing" mode and then try accessing the home directory of your old user on the hard disk? Does this also lead to that system hang?

I have not tried Ubuntu without installing or had ever tried to boot Ubuntu from usb . But I have something to tell . I have a working kali Linux usb stick with persistence fully working ok . From there , I can access the home directory or any directory or anything even root files of my old user , new user without any problem .I can move or delete whatever I want using kali stick . Does that help

I have also found that these weird messages come only when I tried to login from old user but for new user , no messages come whatsoever . Something more , these messages come for about 550 seconds .

If its the same issue then please close this as a duplicate