“We think somebody else might be using your account”, no password for linked account and insufficient information provided to questionnaire

This is the second time this has happened to me at a crucial stage where I am waiting for job application responses.

I tried signing into my account and it says they think somebody else might be using it. They give me an option to send my verification email to an email address so old that I barely even recognise it let alone remember the password. Or there's the 'I can't verify my account' option where you fill out a questionnaire. On the numerous times I have filled this out I receive a response saying the information I provided was insufficient in order to regain access to my account. I don't know what else to do and would strongly appreciate some help or guidance to my other options.

When I am next able to sign in I will be sure to change all my security information and make an alternate email address so that if this ever happens again I won't go through this stress over and over again.

Please help.