How does an attitude indicator know which direction is up?

OK, I get that the thing has a gyro in it which remains rigid as the
airplane banks and pitches around it. But how does it get "reset" to
the correct orientation?

Is it dependent upon gravity to orient it when it spools up, and the
plane is on the ground?

Conversation started when I was talking to a pilot on a dual-vacuum pump
powered 172. One vac pump is shot, and it did not build up sufficient
vacuum to spool up the gyros until airborne.

Well he was VFR only, but still...