1) What console are you using it with? GBA, GBA SP or DS Lite? Please note that it only works with original hardware, and not with cheap Chinese clones.
2) If you don't insert the micro SD in the cart and turn on the console, does it show a white screen with the message "no SD inserted" or something like that? (see photo below). If the black screen appears, and you are using real hardware, then the cartridge is defective.
3) Otherwise, try removing and inserting the flashcart into the console slot again pushing It well, so that it makes good contact, and try it again.