how do you know if the good oil has gone off?

I decanted some extra virgin into a smaller bottle (sterilized dark bottle) to take on a picnic BBQ party last autumn. Didn't use any but kept it in the cellar all winter.

Poured some on bread to grill today and it tasted gross (very bitter)!

What happened?
Can I resurrect it?

I know my reply may be a bit late haha! But here goes:-

There's no way you can resurrect oil that has gone rancid. Throw it away. Oh and maybe you should try palm oil. It is unsaturated, heart-friendly and full of antioxidants and vitamins. Plus, it is stable in frying and bland so it doesn't give that weird taste you may get with other oils. For more info you can go to www.palmoiltruthfoundation.com. I go there for my recipes