router cant reconnect to any network

hii all, i have a xbee zb router connected to my cordinator. if i shut down my cordinator fr a long period of time(say 9-10hrs) then the router is unable to join it after i switch on the cordinator. distance have no role in this as they are only few meters away of each other. i hv set the pan id to 0 in the router, NW & JV are 1.
please someone help me out to understand this problem & find a suitable way to resolve the issue.
thanks in advance.

Try simply issuing a Global network reset from the coordinator before you power it off. (ATNR1).