If you are attempting to log in on the HCS Voice Packs website, that would be something to contact them about, via
their forum, or
their Discord.
An email address and a contact form are also made available by them
here.
If you are referring to retrieving your VoiceAttack license information (rather than any third-party HCS Voice Packs license information), that would have been emailed to the address used for the PayPal transaction from registration_only_do_not_reply@voiceattack.com.
If you cannot find that email, you can fill out
this form to have it re-sent to that address.
This only applies to the website version; if you purchased VoiceAttack (again, VoiceAttack itself, not third-party HCS Voice Packs products) through Steam, your license is validated using your Steam account when VoiceAttack is started.