I am receiving a message stating that my account is "Either still logged in, or the last world you were using has become unavailable. Please try again later."
Your account has been active within the past three months.
If it wasn't you then you have to assume a hijacker is using it.
Try following the instructions on the
Password Reset
support page.
Basically, that page guides you to set up a Jagex Account if you haven't done so already.
You can set a
new login
at this point. Once complete you will be able to hit the 'End all Sessions' button in Account Management and bump the account out of the game.
The hijacker won't know your new login or new password and won't be able to get back into the account.
If you really don't want to upgrade to a Jagex Account right now, follow the instructions under
"If you don't have access to your registered email"
on that page.
You will be able to set a new password which, hopefully, should also bump the account out of the game.
But you must do a thorough security review on your device and email because you need to find out how the hijacker discovered your login AND password.
Instructions:
Security tips