I had a problem with one of the automations, where due to a problem in the service client we are not able to open the automation. We have already redone the automation but the same problem occurred later.
If you are using the ServiceClient, it creates a dynamic assembly that needs to be present on your machine. If you have access to the machine and user who created the automation, the file should be located in %AppData%\Pegasystems\DynamicAssemblies. That file needs to be present for any developer to open the automation that uses it.
Unfortunately then, I believe you'll need to re-build that automation. Make sure you are looking in the correct directory though. %AppData% is specific to each user. If you have access to that machine, but another developer was logged-in, then you'd need to search in their folder.