Hii!
Since you sat for the class 12th exam in the year 2017, you can go for either of the two options. Firstly, you can file an application and request the respective board of education to grant you the permission to sit for a re- exam and give the exam to pass it, or, you can apply in class 12th as a private candidate such as in NIOS or any of the open schools present in India. You can pass class 12th from there too.
Hope this helps.
Good luck!