Did you get below freezing temps a couple of weeks ago? I know there were freeze warnings all across southern California including at my mom-in-law's who lives near the coast.

Bougainvillea is frost tender and gets heavy damage at very cold temperatures. Some years ago we got snow at my house (I'm close to the mountains) and it killed my bougies.

Large mature plants that are well established can survive and grow back but small ones don't fare so well.

Another possibility is gophers. They killed another of my bougainvillieas.

To Dis's comments: I had a Bougie planted in the open, but by a huge rock that offered some protection for the roots and base of the plant. The top died-back almost every winter, but resprouted in the Spring. I suggest you give the Bougie time... maybe until June even... to resprout, and then prune-away what has died over winter.

To Dis's comments: I had a Bougie planted in the open, but by a huge rock that offered some protection for the roots and base of the plant. The top died-back almost every winter, but resprouted in the Spring. I suggest you give the Bougie time... maybe until June even... to resprout, and then prune-away what has died over winter.

The cold kills them, they could come back, but no guarantees.

I suspect it is the cold. However, since the boug has been growing prodiguously for the last 15 years, I am a bit stunned by its sudden setback (I hope) due to a cold snap.

I suspect it is the cold. However, since the boug has been growing prodiguously for the last 15 years, I am a bit stunned by its sudden setback (I hope) due to a cold snap.

If the roots are well-established you can cut it back hard nearly to the ground and it should all grow back afresh.