Hi Pooja 

To apply for NEET exams most important thing is to get your 12th exams cleared .There's also one more important thing which is minimum percentage acquired by a person .

If a person belongs to unreserved category he/she needs to score minimum 50% marks in 12th whereas for the reserved ones like SC/ST/OBC minimum score required is 40% and 45% for physically handicapped.

As per your question if  you have  failed in any of the semesters in class 12 but got your 12th board cleared then you can apply for NEET exam but if you have not  yet cleared your class 12th then  I'm sorry but you can't apply for NEET UG exam

Hope this helps