How do I keep my chocolate ganache in the right consistency?

I made a chocolate ganache and after a few hours in the fridge it was in the perfect consistency. I left it out of the fridge so it would be room temperature, and an hour later it became much harder. I tried putting it back in the fridge but it didn’t help. I need to use the ganache on a cake in 12 hours. How do I get the ganache back to the right consistency?