The first thing to check is if there is an entry for that candidate in the "Candidates" tab of your dashboard. If there isn't, the invite did not get submitted and you need to invite the candidate again.

If their entry is there in the "Candidates" tab, you can click on the ">" on the right end of the row for that candidate, you will be able to see the email status for that candidate, for e.g. if the email delivery failed.

If the email delivery was successful, it means that the email was sent from the Adaface servers but it might have hit the spam folders or an IT security trigger, especially if it is a corporate email id.

Within the same option ">", you can also copy the invite link for that candidate and send it to them directly.