The Ps logo is gray by design in Photoshop CS5.
However, being unable to create or open files is out of the ordinary.
Do you have a display with 1024 x 768 pixels or larger? It's a known issue that the Save dialog won't come up on displays that are smaller than that.
If that's not the issue, there have been several other users who have reported issues opening or creating files in recent days...
Unfortunately I don't think anyone's been able to get to the bottom of what the various folks who are experiencing these failures have in common, or even come up with a workaround short of rebooting Windows. You folks all need to list what hardware and software you have to try to find out what you have in common.
Thanks for your help Noel. I rebooted and all is OK now! I should have thought of that simple solution myself.