you probably have something changed in your Color index that shouldnt be. go to eimage->mode->RGB color (or something like that)
Well, i tried to open a .PNG format render, by going to file>open as and it said that the .PNG format isn't valid in the photoshop, but just yesterday, i was rendering pics and saving as photoshop....
that happened to me, if you install a image program, sometime you might of set it to opening in that specific program by default so that prgm kinda takes it over. do your .PNG files have a different icon or anything?
In photoshop, i can't open .PNG unless it's open as, and they have the regular icons as before. Also i can't save as .PNG