HP Phoenix computer can't detect second monitor

HI,

I have used this for a while (18 months) on one monitor and wanted to extend the desktop to a second monitor - but the system fails to detect a second monitor when it is plugged in.

It has an AMD Radeon 6850 with two DVi slots - both monitors work as I can swap them over.  But neither the auto detect (HP Catalyst) or the manual detectoption  show up a second monitor.

I have updated the ADM driver, restarted etc etc........ not sure what else to try.  Any suggestions?

thanks

Mike