I am not sexually active and missed my period for 5 months. i have a dark brown vaginal discharge. what can it be ?

Many things: There are a variety of conditions which can cause vaginal discharge. It may simply be spotting or it may be a vaginal infection. There are several vaginal infections which are not sexually transmitted. Missing your period for 5 months is also not normal. Take a pregnancy test today to rule out the most common reason for missing a period. Then go see your doctor.