I am having a similar problem. Last night all A-OK. This morning:

0 DB_ERROR_NOT_CONNECTED
in:
[select * from project_version WHERE project_version_key = 'Zen-Cart Database' ]
If you were entering information, press the BACK button in your browser and re-check the information you had entered to be sure you left no blank fields.
Host swears its not them (don't believe them). Front end of store is fine. Only admin is the problem.

Tried to go to myPHP admin and repair as per an Ajeh post, but couldn't locate the REPAIR functionality.

OS: Linux
Apache: 1.3.37 (Linux)
PHP: 4.4.4
MySQL: 4.1.21-standard


Thanks & Happy Holidays!
Laura