While PlayStation Network is one of the most stable platforms out there, it gets some issues from time to time. Recently, many players are reporting that they are getting the error code CE-42739-5 while using their PlayStation consoles.
The reason for that is that the PlayStation network is down. So players are not being able to access their games or use online functionality in many of the games. Some players are also not able to see their games in the library altogether. This will likely be fixed very soon. You can check the status of PSN servers here.