Thanks for your query.
Since you have missed period although you had regular periods before that means there is some issue.
Three negative pregnancy
test suggest that you are not pregnant.
So the cause of not getting periods can be hormonal
,PCOD(poly cystic ovaries),
and it needs to be investigated.
I would suggest you to consult a Gynaecologist and get investigated for the cause and she will prescribe medicines for the periods to come.