If you've sent a marketing email, all replies will go to the FROM address you selected in the email editor. You'll want to check the inbox associated with that from address.
For one-on-one/sales emails, all replies will go the selected FROM address, as well! If you've sent the initial email from the HubSpot CRM, all replies will appear on the contact record timeline in-app. However, if you've sent the email from your personal, connected inbox and you have not enabled the 'log' checkbox, you'll need to revisit your inbox to find replies. You can retroactively log email replies in HubSpot by forwarding the responses to your forwarding CRM address. Check out this guide for more on how to do that:
Hope this helps! Thanks for reaching out.