Sometimes a microscopic piece of debris will get in the one way valve. To free it you could try a number of things. Fill the gun on the opposite side that you normally do. Fill the line until you hear it click, bleed it, fill till click, bleed, repeat. Last resort - VERY CAREFULLY/Gently, bump the gun during a fill process to dislodge the debris. Don't bump the filling connections you certainly do not want them to pop off!
If it looks like the o-ring is gone, sounds like you're going to have to replace it. It could have been the problem and/or it could have been mashed up when you unscrewed the fill valve.
You could call AoA or Precision Airguns (Jim is super helpful!) to see if they have any they could send you. You could also just replace the fill valve, if you felt so inclined.
While you at it, see if they have any fill port covers. It's cheap insurance against grit. If they don't, you could also look on eBay.
Tom