A simple, but useful feature in WHMCS is re-sending product emails. In this instance "product" almost always means web hosting given that is really the only product you are providing (other than possibly domain names).
The email you would most typically need to resend is the Welcome Email with their FTP credentials and other details.
To do this login to WHMCS and search for the user in the search box in the upper right-hand corner.
Once you have found them, navigate to the Products/Services tab.
Click on the Products/Services tab
After that, you'll click on the More dropdown in the top, right corner of the Products/Services tab and then click "Resend Welcome Email":
If you need to send another type of message, click the "Send Message" link and then choose the correct message from the list provided: