Won't start, backfires

Bike wont start just backfires.
Tried to start it in the early morning cold almost got it started and it backfired, so I gave up and went to work. Tried it again this afternoon and it just keeps backfiring. won't start. Where do I begin to look for a problem/ solution? Good thread?
It's been running fine until now.

Before the cold morning, when was the last time you started it?

Started it up the night before to move it off the street. Rode it to and from work that day,