Look at the device entries under the My Lighting tree. Those that have a Red Icon have not responded to an ISY initiated activity. Right click on the device and select Query. If the device is communicating now the Red Icon will be cleared. If there are lots of devices with the problem it could be the ISY PLM, something could have happened to one of the 120V leg couplers, some appliance or device has started generating noise.
Has the Insteon network been stable for months and this just started or has this been an issue from the beginning?