This is an answer by someone.
Why has his/her username gone? Did he/she leave the website?
The username disappearing as linked by you indicates that the user deleted his or her account. This means all their questions and answers remain on the site for the community to refer to, but their name is displayed as whatever user code or user number was associated with their account. For example if I decided to delete my account it will show the following (user20129 being my user code):
Your questions and answers will remain on the site, but will be disassociated and anonymized (the author will be listed as "user20129") and will not indicate your authorship even if you later return to the site.
In all of my questions/ answers instead of Archit, user20129 will be displayed and you can’t click on it.
You can have a look at How do I delete my account? to access the part which I quoted above and see what shows in case you decide to delete.
PS: please don’t delete your account, even in case you ever decide to become inactive. It doesn’t make sense.