Your camera periodically let's the server know it is online. When the server does not receive such signal for a certain period of time, it will assume that the camera is offline and this will show in the status of your camera.
There are several reasons why those signals may not reach your server. It may be that your camera is turned off. In that case you simply turn it back on again. Another reason may be your wifi connection if this connection is either weak or intermittent.
It may also be the case that your camera appears offline whereas it isn't (this is the case if you still receive notifications for instance).
To see whether that is the case you may want to try two things:
- Refresh your camera list by pulling it down with your finger.
- Turn the camera off and on again.