The phrase "had a flash" has a number of different possible antonyms, depending largely on the meaning intended. If by "had a flash" one means "suddenly had a brilliant idea," some possible antonyms might be "drew a blank," "had a mental block," or simply "had no ideas." If one is referring to a sudden burst of anger or frustration, the antonyms might include "kept one's cool," "stayed level-headed," or "maintained composure." Similarly, "had a flash" can also refer to a sudden memory or recollection, in which case the antonyms might be "forgot," "blanked out," or "failed to remember.