If you are also experiencing other problems with your pregnancy like cramping or bleeding, it is almost always a sign of miscarriage. Your practitioner may have been looking specifically for this drop if you were having other symptoms, or you may have a history of miscarriage or medical complication that made looking at your hCG levels important.
Typically your practitioner will continue to draw your blood until the levels have returned to zero. If you have not had miscarriage symptoms, you may be sent home to await a miscarriage or you may be offered a dilation and curettage (D & C). There are very few reasons why the decision of which is best has to happen immediately. You have time to ask questions, wait and even decide later what you wish to do.
Obstetrics: Normal and Problem Pregnancies. Gabbe, S, Niebyl, J, Simpson, JL. Fifth Edition.