Something very similar happened to me a few years ago. Lost hearing in one ear completely. Went immediately to ENT specialist and they tested my hearing and I was basically deaf in that ear. Long story short - certain virus that needs immediate injection of steroids into ear asap. They thought it had been too long for me to have the shot but put me on heavy steroids immediately. 7 days into steroids and my hearing came back! I asked what had had happened since the period of time from when I should have had the shot was over a month ago and the ENT Doc said "sometimes things happen that cannot be explained but maybe we had enough time to save your hearing!"