ORA-01034: ORACLE not available

Symptom:
When attempting to start (or alter) the database the following error is generated:
ORA-01034: ORACLE not available

Possible Cause and Remedy
Oracle indicates the following possible causes:
1. The SGA requires more space than was allocated for it.
2. The operating system variable pointing to the instance is improperly defined.

And ORACLE recommends the following remedial action: Refer to accompanying messages for possible causes and correct the problem mentioned in the other messages. Retry after ORACLE has been initialised. If ORACLE has been initialised, verify that ORACLE was linked correctly. Also, refer to the installation or user's guide for additional information about this error.

1. After exhausting all other options, a simple reboot has been observed to resolve this error.