Sometimes a spoke just breaks. It could have had a manufacturing defect and just let go. Or something could have hit the spoke. I was riding on a trail in June and a large, solid stick jumped up into my wheel and broke a spoke.

Is it safe to ride? Often when one spoke breaks, you just replace it and true up the wheel. Off you go. However if a second or third spoke breaks, well, it might be telling you something.

I mentioned earlier that I had a spoke break in June. I was crossing Missouri on the KATY trail when it happened. I had spare spokes, but wouldn't you know it, it was a spoke behind the cassette and I didn't have a cassette tool. So I widened up the brakes and loosened the spokes on each side of the broke spoke a little. I rode it that was for about 70 miles when a spoke 180° opposite broke. I then rode the bike that way for another 60 miles until I found a bike shop that replaced the spokes in Defiance, Missouri (a great little bike shop). I've ridden that bike since with no issues. BTW: It was a 36-spoke wheel which probably helped matters a lot.