Dissociation. If you do this a lot - would have it medically evaluated.
Zoning out. Zoning out and blank stares can be a lot of different things. We all sometimes get tired or fatigued and everyone's attention can wax and wane to some degree. However, this can also occur due to metabolic changes, medicatoin effects, anxiety, absence or petit-mal seizures, blood pressure changes, etc. More questions and examination by a doctor should start the path to an answer as to what it is.