PM's not opening and Hidden

I don't know exactly how to explain this, but I'll try my best..

I recieved a PM, and when I clicked on it to view it, it just went back to my inbox, not the actual message..
It's really weird. All my other PM's that I've recieved opened fine.
And I even tried opening it on two different computers, so it's not just the computer acting up.

and how do we become hidden?

look in prefferences