Please go through the failover documentation. Moreover, you haven't give details on how you have implemented replication either using perl tools or without it.
In case of Slony with perl tools, you get inbuilt scripts which will help to do fail-over when the primary server is NO longer available (ie in a Disaster).
I try to replicate two server i.e. 1 master and 1 slave using slony replication. can please explain me that when my master server is fail then how to my slave server is work as master (means it able to perform DML operation)? How to do this please give me solution for that?