Various causes. Abrupt speech arrest can happen when brain processing is disrupted. This may happen with stroke but also seizure, migraine, acute pain, sleep attack, or sudden distraction. If stroke there is usually associated weakness face and right hand.
Speech arrest. This symptom can occur in a variety of settings: stroke, transient cerebral ischemia, fright, sleep attack, partial epilepsy, heart beat irregularities, fatigue, worry. There probably other causes not named. But if this has happened to you, don't wait because it may not be a stroke, since many of the other possibilities can be rather nasty too.
Stroke not only caus. While stroke is probably the most common, other injuries to the speech centers of the brain could cause sudden loss of speech. These include infection, trauma and tumor.