I haven't used Oommoo, but I'd recommend letting it warm upstairs. If that doesn't do the trick, then see how deep the hardness goes. Try to poke through it, as maybe only some of the stuff at the top has hardened. If that works, then toss out the hardened part and strain the rest into a new container.
I had a similar thing happen with Polytek Easyflo 60. Both part A and B were in the same "ketchup squirter" type containers, but part A got cloudy and hardened a bit. I just strained it with some pantyhose I borrowed from myself, into a new container. Either something contaminated it, or it had some sort of photosensitive reaction...