Does this happen with different images or is it just one particular image that wont play ball?
I still can't replicate the problem so If it happens with ANY image you open and not just one in particular then I'd suggest resetting the preferences.
Make a backup of your preferences folder first
Close PS.
Make Backup!
Start PS but IMMEDIATELY HOLD Ctrl + Alt + Shift.....before you see any splash screen. (You may even be able to start PS whilst holding down those keys....try it)
At the small 'Preferences Reset' window click on Yes, or Ok, whatever it is to reset the preferences.
If this works then go with it.
If not, close PS and restore your backed up folder....and we'll have to think of something else.
Preference (settings) folder location for Windows is:
C:\Users\< User Name>\AppData\Roaming\Adobe\Adobe Photoshop CS5.1\Adobe Photoshop CS5.1 Settings
...or variation of depending on your OS and CS5 version.
Copy the WHOLE " Adobe Photoshop CS5.1 Settings " folder......and then do the Ctrl Alt Shift thing.
It may be worth trying this before anything else just to rule it out if nothing else.
Regards.
MrToM.
EDIT:
I forgot to mention you should really close PS first, then make the backup....sorry.
It shouldn't make too much difference anyway but just in case.