So are you talking about paying for clicks? Since your post in the the PPC section. It could be your keywords or saturation. You may need to wait longer. Sometimes ads appear stopped for hours to over a day, just because nobody searched that term. If it is a term related to something an adult would search, then you may get no traffic except after work hours (like 4pm-11pm)... Something for college students may be at night, or at least after noon.