I bought my first Bonsai - a privet in need of TLC.

[Melospiza]

Did you have a hard freeze that night? Chinese privet keeps its leaves for me unless the temperature drops below -4 C or so. I also wonder if you gave it enough help after pruning the roots. Privets are tough, but when I do heavy rootwork, I make sure to give a lot of warmth and light so their roots and tops can grow strongly and recover.