Don't worry, the money isn't lost! If someone sends a payment to an email address that isn't already signed up on Venmo, then you can claim those funds by adding that email address as a second email address in your Venmo profile. Venmo has official instructions on how to do that near the bottom of this page, under "My friend paid me on the wrong email address". Once you add the email address as a second email address, the funds should automatically appear in your Venmo account.
If you're still worried about the status of a Venmo payment, then the person who originally sent the payment can always review the payment status or cancel the payment entirely from the Venmo app. You can find instructions on canceling a Venmo payment here.