HI HomerJ,
The issues could be caused by a number of things. Since it is affecting multiple users and you are able to get to the main log in page on the guy, I would check the following:
See if you can ping the DB server from within the system (especially from the application server), as it sounds like something has failed in connectivity from the application server to the DB server (if the DB server connectivity is lost then QC will not operate).
If you can successfully ping the server and connect to it from the application server then I would check that the user id (especially the qcsiteadmin user id) can connect to the DB and that the password hasn't expired or changed. I would check with the DBA to verify this and to insure that you have the proper permissions.
Try these and see if it helps.
Dan