Forgetting a password is a common thing and the solution is usually simple. On Yudum.NET account recovery runs through e-mail: a link is sent to your registered address, and you set your new password from that link.
The trouble starts if you didn't give an e-mail address when you registered. In that case there is no way left to verify the account. This isn't strictness but a security requirement: handing an account with no e-mail address to whoever says "it's mine" would mean handing it to anyone who asks for it.
If someone is using your name
If you remember your password but your name is being used by somebody else, that is a separate situation and an easy one to solve — it is covered in the steps below.