This error occur when connecting remotely when a firewall terminates the connection.
To diagnose this you need to Check related OS PID is Still exists..
you can increase the value of the sqlnet.ora expire_time parameter in the sqlnet.ora file
or to check for a expire parameter in a SQL*Plus profile
or setting the parameters sqlnet.inbound_connect_timeout and inbound_connect_timeout_listenername to 0 can stop the ORA-03135 error
Check to see if there is a Network Address Translation (NAT) between the client and server
In Windows, check to see if a Windows firewall is checking for your local connections:
Windows XP -> Control panel -> security -> Tab "Advanced"