I think you meant you got a very low score on one of these exams instead of failing because you can't fail these exams. Schools look at the scores of these exams as one of a ways to weed out the "weaklings". Usually, low score means low chance to get in the school. However, it's not always the case.
I took my SATs in my high school, one in my junior year and one in my senior year. I got a 900 in the first time and 1000 in the second time, which was low compared to my cousins who got 1200-1300. I later applied for college and still got in even though my SAT's was low because my GPA was good and my cousin wrote my personal essay for me
. She writes very well.
My cousin took her GRE for her grad school last summer. She told me the score but for the life of me I can't remember it now. All I can remember is she said the score was low. Still they accepted her into the program after a personal interview with her.
My point is that you can always study for these exams if you wish, yet the score is not the only thing you'll get accept in the school you're trying apply. Your overal achievement, personal essay, one-on-one interview, etc. can also determine your admission. Try your best. Get a score in the range they require and don't forget to polish your other things also.