Might be time for a factory reset of that phone. I had a few nuances and strange events like that from time to time, especially in the 2nd year i owned my GNex.
Yes, after trying to figure it out on my own...I did a factory reset on the phone. Although it now runs smoother, the camera still will not open. I would upgrade by I'm grandfathered in with unlimited data.
You could always use a 3rd party camera app too. Just my opinion, but after the dismal battery life i got on my Gnex, plus it being fairly outdated at this point (3 phones ago for me) I might opt to buy full price new from Verizon or go to Swappa.com to buy a good used much cheaper and newer phone....and you'd still keep your unlimited plan.