The important rule is that you should complete your 12th and pass in all subjects scoring the required minimum percentage in Physics, Chemistry and Biology/Biotechnology. So if you wrote your 12th this year but failed in Chemistry. You can write your Chemistry improvement exams and if you pass and get the required score in Physics, Chemistry and Biology/Biotechnology, you can appear for NEET next year. You should score 50% if you belong to General Category and 40% if you belong to OBC,SC,ST.