The following is an excerpt from page 21 of the Warhammer Annual 2002 release with an expanded explanation by Gav Thorpe regarding how to handle loss of war machine crew.

War machines that suffer crew casualties fire at a slower rate (normally when reduced to a single crew member). This takes effect after the next time it is fired. In other words, the war machine can fire again one more time before having to take extra time to reload. For example, an Empire cannon is reduced to one crew man in the enemy's third turn. In the next Empire turn (say they went second, so it'd be their turn three), the cannon may fire normally. It will miss Turn four to reload and may fire again on Turn five. These extra reload turns are cumulative with any turns missed due to misfires or other factors. Following the previous example, on Turn 3 the Empire player rolls a 2-3 on the Misfire table, so that it misses two turns shooting (three including the missed shot that turn); and so will not fire on Turns three, four and five!