problem resolved.
Seems the issue is after restarting Quality Center application server, the details of the "user" within the system that run the qc services has been altered and it has been confirmed by HP helpdesk that it is common things to happen. Which is a bit of a worry.
However after changing the user details back to what it should be (administrator user which was used to set up QC initially), and restart the services, QC seems to be back to normal again.