Two things.
Get your email secure.
Usually an insecure email account is the cause of a hijacking. Change the password for it, enable 2FA, and check activity logs.
Get your RuneScape account secure.
- Head to this support page: https://support.runescape.com/hc/en-gb/articles/360001429138-Change-password-
- Do you have access to your RuneScape account? Yes
- Do you have access to your registered email? No
- Then hit Account Recovery and fill in the form to prove your ownership of the account. With this form you'll be able to register your email address.