Helpless

The term “helpless” is used in multiple places throughout the books, including in the description of stinking cloud. However, “keyword mechanics” were not really a thing in AD&D, and so some interpretation is needed to establish a defined meaning for the word. Since helplessness is very deadly, it is critical that everyone at the table has a solid definition.

Non-magical sleep is a type of “helpless”.

Helplessness is not quite as bad as being held (which includes magical sleep), because a helpless creature can often still move.

Attacks against a helpless creature

An attacker targeting a helpless opponent during an active combat automatically hits.1) DMG 72.

However, if the attacker is classed, they my choose to make an assassination attempt, regardless of their class. In such a case, regular weapon damage is still applied on failure. So unless an attacker does not want the target dead, this is always the better option, since it automatically hits as well.2) DMG 75.

Movement when helpless

Depends on what caused the helplessness. In the case of a grapple, of course the target can't use their MV. But in the case of stinking cloud, nothing is restricting movement.