I know this is an old thread, but my L830 capacitive buttons stopped working over the last few days too.
Found a lot of threads with the same problem, which made my heart sink.
However, a found a suggestion from Microsoft to the problem: take off the screen protector if you're using one. I did, and it solved my problem.
Maybe grit or dust gets under the screen protector preventing a good contact. Hopefully my experience helps someone else.