Xbee transceiver does not response.

Hi

I am using xbee transceivers (PRO-S1) to build my own sensor network. I setup the network, and everything was working fin. Unfortunately, those transceivers stop responding although they are shown on X-ctu, I cannot read, write or restore them. I have tried all the possible solutions such as restoring, updating the firmware, but nothing works. It seems those transceivers went to sleep mode.

If anybody have a solution, please help me with that because I lost almost 4 transceivers.

What voltage level is your sensor running at and did you connect the module direct to the sensor without any sort of buffer circuit?