Contact the company/administrator that runs it, explain the situation, and see if there's anything they can do.
You might first see if there's a FAQ section; there may be a specific procedure the administrators would like you to follow.
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Click on the 'Forgot Your Password' link next to the login area, and follow the instructions listed there.
if you go to the runescape homepage and type in the small search engine located at the top, type in recover account, and it will come up with a list of options, good luck :)
I don't think you can, but maybe check the email from when your account was made..?
I don't think you can do that.
Go to "Recover gamertag" when selecting an account. You'll then have to enter your gamertag, the email you used for it, and the password.