Hi, Robin.
Microsoft IME is good at getting stuck sometimes. It usually seems to be due to an accidental keypress that you can never, ever find a second time.
I think I got out of a similar situation using these steps:
1. Set your IME to hiragana input.
2. Type a capital A. For me, this would show a full-width A, with an underline under it indicating I could 変換 to something else.
3. 変換 the character to a half-width A.
This seemed to clear the problem for me. I have a Japanese keyboard, so I have a dedicated 変換 key. I don't know what key you would use to 変換 if you are on a western keyboard. The space bar might work, or it might not.
Good luck!
Jerry Pietrzak
Beppu, Oita