I recently got a ficus bonsai and exposed it to one night of frost. It
since lost nearly all the leaves (some 5-10 are green and left). What
shall I do? BRBR

Don't panic. I visited a bonsai grower in Israel. He was in Zone 9 or 10, where
they get frost occasionally. He said his Ficus bonsai survived frost, but
branches were occasionally killed back. Keep it in the right environment: cool,
not too hot or cold, some sun, don't feed it until it is really growing, and
don't water too often. Keep it a little on the dry side for now. It should
recover, although you may lose some twigs. Don't prune it until it is up to
speed & you know exacly what has survived.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)