Have you reviewed your email, device and account security?
The
Security tips
will help you with that.
Did you find your password changed or still the same?
Was your Authenticator still active or had it been removed?
Was your Bank PIN removed?
Answering these questions can provide clues as to where the weak point was that allowed the hijacker to gain access.
I strongly urge you to upgrade your account into a Jagex Account.
Jagex Accounts have a much stronger security protocol than the old RuneScape accounts.
Information:
Upgrade your RuneScape character to Jagex account
Can't do much about it if your account isn't secure - bank PIN, good password, two-factor authentication, etc. This is why Jagex stresses this stuff. So if you get hacked, it's on you. Malua has some pretty good tips there.