Depending on the nature of the investigation, it could end your investigation or seriously slow down your investigation, because you would then, potentially, have to try to find some other way to gain access to the information that's missing from your investigation.
Once again, it's a chain of evidence that brings you to your charge in this case. Police officers don't give up very easily. We often look for other sources of data that might be able to produce that information, but at times, if there's no other source, it could very much end your investigation.