Would need very specific facts concerning this issue to provide any definitive answer, so I'll try a general answer. Evidence lost by the prosecution (or defense for that matter) is not the equivalent of it never having existed. Witnesses who are competent to testify to the fact of the evidence can normally do so. The inability of the prosecution to produce the actual piece of evidence will go to the weight given the fact finder (jury or judge) to that evidence. So, for example, if the lost evidence was spent cartridge cases from 38 cal. bullets that had originally been found at the crime scene but were lost before trial, that would not preclude the police officer who found the at the scene fro testifying to that fact. The state's inability to produce the actual brass casings would be argued by the defense as casting doubt on the officer's testimony about what he had found. Their absence from the trial would not preclude the officer's testitmony about recovering those items.