Hi ,
There must be a communication error(Network issues) with client server,check the "alert.log" files more on this issue".
check for network problems and review the SQL*Net the server process might be dead and whether a trace file was generated at failure time.
Webserver or listener process is not running
Missing entry to the "tnsnames.ora" file check the port number.
Network issues
Problems with connection pooling (shared servers, MTS)
Thanks,
Brindavan M