If someone asks you a question that stumps you, tell them that you'll figure it out later. Your brain is working fine, but it is working a bit slowly today: so you might need to ask for more time than you usually need to complete certain tasks. Your brain power ebbs and flows just like your energy level. There's no miracle food or exercise that will help get the synapses firing faster: it's just a normal, temporary slowdown that shouldn't cause you any real problems. Try not to let it bug you.