If you have reporting turned on for a media player, it will generate report data while the player is offline. However, any data that is collected on the player while it is offline will not be visible in the Reports section of AerWave until the next time it comes online. This is because the player needs to communicate with the AerWave server in order for the latest report data to be viewable.
If your media player is in a location with only intermittent network connectivity, there's no need to worry. It won't lose any report data while it's temporarily offline.