Over which sales channel did you buy your ticket? Generally, the tickets from a ticket machine and from the counter are not personal and therefore, it would be really hard to get a reciept or something similar if you have lost the original of the ticket. For those tickets, there also is no confirmation e-mail. If you bought it via the webpage or the app, the ticket was personal and therefore there must have been sent a confirmation e-mail. Other than said e-mail, there is no way to directly obtain the ticket. You can try to contact SBB themselves through the Contact Center by phone, but with no guarantee will be able to help you:
SBB Contact Center.
Tel. 0848 44 66 88
(CHF 0.08/min.)
(available 24/7)
From abroad: +41 848 44 66 88
I unfortunately fear that there is simply no record anymore of that ticket.