If you are trying to log in on the desktop site (on a computer) you are likely running a VPN or proxies. Both of which cause the identical issue you are discribing. So, you either need to disable any and all VPNs and proxies or add exceptions for POF in order to log on.
If you are having this issue using one of the apps then I cannot help you.
If you still get emails about matches and messages then your account is still active. If you don't then POF might have deactivated it.
In any case try requesting your password: https://www.pof.com/getpassword.aspx
Submit only the email address that is assigned to your actual POF account. This will actually reset your password by by sending you an email with a link to visit to enter a new password.
Check you spam folder for the email. If you don't get an email from POF after this then most likely your account has been disabled or deactivated.