You can try submitting a claim via the
Lost items
support page though they do say that items lost due to a hijack or scam are not covered.
The main thing after a hijack is to try to work out how it happened and what you can do to prevent it from ever happening again.
The main thing nowadays is to upgrade your account to a 'Jagex Account'.
Jagex Accounts have enhanced security that makes it extremely difficult to get unauthorised access.
Also, check your 'Linked Accounts' tab and unlink any accounts you see linked. Click on 'Manage Steam' to check for a linked Steam account. Linked accounts are a real weak point as they bypass Jagex security and, instead, use whatever security the player has going on the third party website.