It depends on what other evidence there is. In a criminal trial, the victim is just a witness--an often-important, sometimes critical one, but still only a witness to the crime. The case can proceed and the defendant be convicted if there is sufficient other evidence. For example, take an assault case: if there was some or more of other witnesses to the assult, video/security camera footage, medical reports or X-rays or photographs showing the injuries, the defendant's DNA or fingerprints on the victim or a weapon, the defendant's ID found at the scene of the attack, etc., the defendant could easily be convicted even if the victim him/herself cannot be found.