What sorts of problems make you feel light-headed or unable to think?

Many possibilities: Transient inability to think can be associated with emotional stress, memory issues, speech issues, and even epilepsy. If one has migraine, these problems can occur in that syndrome also. Panic reactions can take a variety of forms, and sleep deprivation may cause both the light-headedness and problems with thinking. Adverse rxtns with medicine should be considered.

Multiple: Several possibilities come to mind, including anxiety -- either temporary like having to give a speech, or more chronic like an ongoing anxiety disorder such as generalized anxiety d/o or panic d/o. Intoxication with alcohol or other substances is another. Dehydration and medications which lower your blood pressure -- and standing up quickly with these on board.