Why is my Appature F - Stop stuck at 0 ?

I have a Nikon D7000 only got it a few weeks ago and am still adjusting to it. As far as I know I've done nothing out of the ordinary with it but when i turned it on the other day I found my F stop was stuck at 0 and i am unable to change it, I have tried in all modes (i usually use manual) but it wont budge, there is like a little triangle that looks like a steep hill sign before the F which is not usually there. the shutter speed and everything else can still be adjusted fine. I have looked through my manual but can't find anything helpful. PLEASE HELP ME :-(