I have had that happen.
Try your preferences and see what it is set at.
MAKE SURE CAPS LOCK IS OFF.. That bites everyone at one time or another.
If that fails reset the preference files/
control+shift+alt just as you open PS, it will give you a prompt to reset, say yes.
Need to add:
If you reset the preference file, be sure to go check all your preferences and make sure they are as needed, color space etc etc:>))))