Is this a recent purchase?
Have you just put a lot of media on it? If so, it may be busy indexing and thumbnailing the data. It shouldn’t make it disappear, but it can make it very unresponsive. It shouldn’t really, but I’m afraid it does… It may be worth just leaving it to do its thing for a few days (yes, it can take that long…)
When you say you’ve power cycled it, did you shut it down first, or just pull the plug? Pulling the plug on a NAS can cause trouble. And it will restart the indexing, and may need to do a disk scan to check for problems caused by the power removal.
Have you tried either a 4 second or 40 second rest (see the user manual for details)?
Are you sure the cable and router port are okay?
I know it’s frustrating, and we’re only offering ‘have you tried this?’ questions, but it’s quite difficult to diagnose exactly what’s going on.