Look for trace files in the background_dump_dest and check the alert.log.
You might find some trace files for the smon or pmon process indicating the real problem.

Mostly this error can be caused by the following:

1. A shutdown abort
2. Incorrect permissions of BDUMP, CDUMP or UDUMP directories
3. Missing datafiles
4. Renaming files when Oracle is down
and some times due to OS issues as well.

If you are trying to do any DB upgrade then follow the supporting documents.