ORA-03113: end-of-file on communication channel

Faced one problem :
When execute one function in Oracle7.3 ,
The error msg : ERROR at line 1:
ORA-03113: end-of-file on communication channel


Any Xpert Please explain what happened?

Thanks

1. The oracle is running on one NT4.0 server
2. The Function was running well before crash
3. Any SP/function with file operation will be failed with ora-03113 msg
4. Any SP/function without file operation can be executed smoothly.