No tengo preguntas = I have no questions. No preguntas = Not questions. The sentence "no preguntas" DOES NOT have the same meaning of the English "no questions" as in "there is no questions"... it means "not questions (as opposed to...)". For example "necesito respuestas no preguntas", meaning "I need answers not questions". If you want to say "no questions" you have to say "ninguna pregunta" (in singular, never in plural, the word "ningún/ninguna" means "not one" so any noun following "ningún/ninguna" must be in singular). I hope this helps..

to expand Mario's answer, in some countries it's common to say "sin preguntas" instead of "ninguna pregunta".