I know exactly what you are talking about. It happens to me regulary. I agree with what the others have said. Think about what your feeling and what you would like to do at that moment, then go and do something,that you enjoy.My problems of blankness arrive often ,when i am asked a specific question, it is'nt that inside my head i dont know the answer,I seem to panic about the best way to relay the answer. Perhaps it is a fear of not being intelligent.