how reset zigbee externally?

I m doing project on ZigBee which are used in RF energy meters. Before 2 years ago when this meters installed in field that time coordinator gives readings of all meters.That means communication between coordinator and end devices is very good.But as time passes the working ratio fails down up to 20%.Coordinator not communicate with the end devices i.e.other meters.
So I want to find solution on this problem for that I was do simple experiment on XBee s2 models.And when same experiment is try on zigbee models used in meters this is not worked.So now I confuse to what I do in next step?
Also I think that,If we reset the zigbee then it is possible to on modules which are going to sleep mode and take their reading.But I not get any information about that.Is there is any way to connect external circuit to reset our zigbee? If yes how to do this? The circuite requires which parameters?
So if u known about this,or any information then plz tell me.

I think you mean Restore. If so, then No, Digi does not offer a hardware method of triggering a restore from a Pin. It can only be done using the ATRE AT command issue to the radios UART.

without doing changes in its firmware is it possible to reset zigbee by using timer or other component.
IF yes then how to do this?

NO. It is Impossible to restore an XBee module to default settings using an external pin.