X-Bee XB ZB assosiation problem

We have X-Bee pan of 25 devices all are configured with 23A7 routers some of X-Bee routers all of sudden not responding till hard power reset

What might be the reason could anyone help me out on this.

You need one of them to be a Coordinator in order to establish a network.

The routers you need to reset, what are they connected to?