oracle一个奇怪的问题

kingoftime3

我已经启动服务了，在使用sqlplus时
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux Error: 2: No such file or directory
Process ID: 0
Session ID: 0 Serial number: 0
在配置服务时，测试出现
Connecting...ORA-12514: TNS:listener does not currently know of service requested in connect descriptor
The test did not succeed.
但我又运行了一次dbca里的configure database ,虽然什么都没做，但退出后一切就好使了。
大侠们教教我该怎么办