I have created an online USCIS account while rectifying FNU on my green card 5 years ago, now I forgot my password and tried to login through ‘forgot password’ link , but didn’t get my answers for secret questions and account got locked. I was directed to fill a short form and gave a ticket number. Now I didn’t hear from USCIS for last 5 days.

Is it okay to create a totally new online USCIS account with a new email for filing N 400 for US citizenship though in USCIS site it says one case, one account?

You should wait for them to fix your account rather than trying to create a new one which their system may not allow with same name, b’day, address etc.