Can anybody suggest me, what should i do? i have forgotten my Fiverr security question!
Only Customer Support can help you with that. We don´t know your security question either.
They probably will ask you for proof of your identity, if you contact them, like a scan of your ID and you holding it next to you on a photo or something, they obviously need to make sure that it´s not someone else trying to hijack your account, which is what the security question is there for in the first place.