Today I was attempting to log in to vSphere server and was greeted with the following message (more or less):

vSphere client could not connect an unknown connection error occurred

After checking all the usual suspects (vsa was accessable, etc) I found the vSpehere service was not running and could not start.
Checking the log for the service (/var/log/vmware/vpx/vpxd.log) I spotted the following:

2012-06-11T09:17:40.166-04:00 [7FFFF3B09700 error 'Default'] [VpxdVdb::SetDBType]: Database error: ODBC error: (HY000) - [unixODBC][Oracle][ODBC][Ora]ORA-00257: archiver error. Connect internal only, until freed.

Our database was having problems, so vSphere couldn’t connect to it. (Too bad vSphere didn’t just say that, it would have saved a lot of time.)  Cleaning up the Oracle DB used by vSphere fixed the problem.