To negate a 被 sentence, you just need to insert 没 or 没有 directly in front of 被. But note that in negative passive sentences, you cannot use 了 since the action didn’t complete. You can use other complements. 昨天的派对我没有被邀请。| I wasn’t invited to the party yesterday. 失踪的小猫没有被找到。| The missing kitten has not been found.