HomerJ,
I would see if this is related to only one project (if you are using an Oracle DB, the project user password could have expired or changed and this would cause it) or if it is related to one user (could be a client or user account issue. If this is not related to one project, I would check the DB user passwords haven't changed or expired. If they are still active, I would check that a connection to the DB is maintained. If the connectivity is timed out then the application will not be useable. You should see an error in the logs as to what is happening with the connectivity. I have seen this before where a DBA was performing maintenance and the connectivity to the DB was temporarily dropped. Restarting the services should resolve the issue.

Try the above suggestions and see if that resolves the issues being experienced.

Dan