Many of them — a lot more than you might think, actually — were still around and doing quite well for themselves! The Reign of Terror was far from a wholesale slaughter of the First Estate. (The death total was anywhere from 15,000 to 40,000, depending on whether you look only at judicial executions or other deaths, such as in the suppression of rebellions — but many of those killed were clergy or commoners, not nobility.) Many nobles either emigrated, or rode out the Revolution on their estates.

In any case, while most nobles had definitely suffered in one way or another from the Revolution, those who survived were hardly impoverished. I wrote about what life was like for the surviving post-Revolutionary nobility in a question last year, and can answer any follow-ups you have.