A:
Before the meeting is called to order, the presiding officer needs to determine whether a quorum is present.
After that point, a quorum is assumed present unless someone notices its loss.
Any member noticing the loss of quorum can make a point of order stating the fact; or if the presiding officer notices, she can announce it.
When the lack of quorum is declared, little can be done.
With only a few exceptions, no motion under consideration may be voted on, and no new motions may be introduced.
In the case you describe, assuming the motion adopted was not of an emergency nature, the vote adopting it is null and void.
The motion can be moved again at a future meeting.
Usually it's too difficult to determine whether a quorum was present when a vote was taken, so the assumption is that it was.
If it can be proved to have been absent, however, then action taken after that point until a quorum was re-established is null and void.
Motions of an emergency nature can be voted on in the absense of a quorum, but must be ratified at a later meeting in which a quorum is present.
For example, if the motion was to hire a contractor to fix a leak in the roof TOMORROW when it's known a big storm is coming the day after, that motion could be adopted at their own risk without a quorum, then brought to the assembly at a future meeting for ratification.
Normally, when a lack of quorum is discovered, the only things that can be done are: