Xbee Series 1 Coordinator Powered OFF and then ON. It Loses Track of End Devices Connected to it.

Background: Xbee Coordinator Association complete. Coordinator Locks to hard coded pan id. Xbee Coordinator Scans through all channels and selects best channel.
Xbee End Device Association complete. End Device searches for Coordinator matching hard coded Pan id and uses the same channel of coordinator.
Scenario: The Coordinator Xbee loses power. Now again after getting power coordinator Xbee restarts. Now the coordinator Xbee may (practically most of the time) lock into a different channel. The end devices are not aware of this new changes in coordinator and uses the old channel to send the data. Thus now the Coordinator will not receive any new data and loses track of all the end devices previously connected to it.

How Can i solve this problem.

The association function of the 802.15.4 protocol was never designed to keep track of an End device as you put it. If this is something you are concerned with, then you need to implement the DA command on your End devices which would then start the association process over again.