Hello Mielaingot28
The red SwissPass card? See a counter agent or call SBB: 0848 44 66 88 (CHF 0.08 / min). A new card will cost CHF 30.
Or the Swiss Half Fare Card for tourists?
Generally you have to contact the point of sale from where you have got it.
If it is an e-ticket with a QR code, you could simply reprint it; you should have an e-mail with a link from where you can download a copy.
If it was a paper document, similarly a paper ticket, it is lost. It cannot be reprinted.
Kind regards,
Roland