The most natural cause for this is a natural disconnection of the target device. The client will naturally appear offline in your dashboard when the target device runs out of power, logs off, shuts down or loses Internet connectivity. There is no need to be alarmed, since the client should reappear online once the target device is powered on again and has Internet connectivity, assuming that the installation option was used when the WebMonitor executable was generated.
If your client does not reappear online after the target device restarts/reboots, then most likely no installation option was used when you generated the WebMonitor executable. However, if you did select the installation option when generating the WebMonitor executable, then this issue may be the result of a bad configuration from a third party application used to tamper the WebMonitor client, e.g. an executable protector.