I've been using Cortana since she the beta started. It's amazing how quickly you get used to being able to say simple things like "Remind me when I get to the office to do X". Yesterday though she stopped working, and only would reply "I'm sorry I didn't get that".
Of course, I had a few friends tell me that it was simply I wasn't nice enough to her, but in reality I found that she was doing the same thing to others! I tried everything short of resetting the phone which from reading other reports doesn't work. What did work was the following that I found on a post here it basically says to do the following:
1.Go to SETTINGS -> SPEECH
2.Set READ ALOUD INCOMING TEXT MESSAGES to OFF
3.Check the IMPROVE RECOGNITION FOR NON-NATIVE SPEAKERS
4.Leave the settings and restart the phone.