This thread has been moved to the Community Led Account Help Forum for best results.
If you did not change your display name yourself, that would usually mean that either:
1. Your previous name was deemed to be inappropriate and was forced changed by Jagex
2. Your account and possibly your registered email is not secure and an account hijacker has changed your name
3. If your account was inactive for quite some time, your name may have been removed in the inactive name releases in August 2014 and March 2017.
Your account shows a long time of inactivity, between April 2010 and October 2021.
It is possible that your display name was removed due to inactivity during that time.
However, if your name was changed
since
October 2021, you would have to suspect that a hijacker has been active and taken the name.
Have you review your security?
Jagex have a list of instructions to work through:
Security tips