Haven't seen that happen or seen similar complaints.  If you did not accidentally delete the return (which would be difficult because you would be asked specifically to confirm that you want to delete that particular return before it's forever gone), your best bet is to contact Support.  Intuit does maintain backup of the data - hopefully, they would be able to restore a copy with the least data lose.

If you do not see the client at all, you may have deactivated the client somehow (note: PTO does not provide any functionality for client deletion at all).  In this case, simply follow the steps below:

1. Click on Clients on the left panel;
2. Click on the View inactive clients button in the top-right corner;
3. Click on the name of the client you would like to restore under CLIENT NAME; and
4. Click on the Make active button in the top-right corner to restore the client.