Evidence can get lost or destroyed before trial and thus is unavailable to the defense. It depends on what that evidence is and what remedy there is for the loss. A primary question is what is the importance of the lost evidence for the defense, does it mean the accused can’t get a fair trial, and so what remedy must the judge apply? It also makes a difference when the evidence was lost or destroyed and who lost or destroyed the evidence.
The issue is very important because by the time an accused gets the opportunity to do anything about it, much time has passed. Investigations take months before anyone in the defense counsel team knows that and more months before the issue can be presented to a judge. This is why at Cave & Freeburg we have started sending evidence preservation requests as soon as we get the case.
The Court of Appeals for the Armed Forces has issued significant cases on this issue which should be a starting point for a military defense counsel in evaluating the meaning and consequences of lost or destroyed evidence: Simmermacher and Stellato.