From Wikipedia:
"In November [2007], Hingis called a press conference to announce that she was under investigation for testing positive for benzoylecgonine, a metabolite of cocaine.
Her urine sample contained an estimated 42 nanograms per millilitre of benzoylecgonine,less than half the level required for a positive confirmatory test for cocaine in the workplace under US Substance Abuse and Mental Health Services Administration guidelines. The International Tennis Federation's report on the matter mentions that "the very low estimated concentration of benzoylecgonine (42 ng/ml) was such that it would go unreported in many drug testing programmes such as that of the US military, which uses a screening threshold of 150 ng/ml." As the amount was so minute, Hingis appealed, arguing the likely cause was contamination rather than intentional ingestion. In January 2008, an ITF tribunal suspended Hingis from the sport for two years, effective from October 2007".
She seems to have been playing competitively ever since the ban ended, so perhaps you have indeed 'missed something' (including three successive Wimbledon titles!).
http://en.wikipedia.org/wiki/Martina_Hingis