If you did carry twins and one of them miscarried, you should still get a positive pregnancy
test and it would also show up in a trans-vaginal sonogram. It could be an early abortion
. Causes could be several and include blighted ovum
deficiencies, infections, drugs etc. Unless a thorough evaluation is done to assess the present status, it is difficult to predict as to what has happened. Please contact a gynecologist for a proper appraisal of your situation and further management. Hope this helps.